Update log requirement to 0.4.1

[dependabot-preview]

Updates the requirements on log to permit the latest version.

Changelog

Sourced from log's changelog.

[0.4.1] - 2017-12-30

Fixed

• Some doc links were fixed.

[0.4.0] - 2017-12-24

The changes in this release include cleanup of some obscure functionality and a more robust public
API designed to support bridges to other logging systems, and provide more flexibility to new
features in the future.

Compatibility

Vast portions of the Rust ecosystem use the 0.3.x release series of log, and we don't want to force
the community to go through the pain of upgrading every crate to 0.4.x at the exact same time. Along
with 0.4.0, we've published a new 0.3.9 release which acts as a "shim" over 0.4.0. This will allow
crates using either version to coexist without losing messages from one side or the other.

There is one caveat - a log message generated by a crate using 0.4.x but consumed by a logging
implementation using 0.3.x will not have a file name or module path. Applications affected by this
can upgrade their logging implementations to one using 0.4.x to avoid losing this information. The
other direction does not lose any information, fortunately!

TL;DR Libraries should feel comfortable upgrading to 0.4.0 without treating that as a breaking
change. Applications may need to update their logging implementation (e.g. env-logger) to a newer
version using log 0.4.x to avoid losing module and file information.

New

• The crate is now no_std by default.
• Level and LevelFilter now implement Serialize and Deserialize when the serde feature is
  enabled.
• The Record and Metadata types can now be constructed by third-party code via a builder API.
• The logger free function returns a reference to the logger implementation. This, along with the
  ability to construct Records, makes it possible to bridge from another logging framework to
  this one without digging into the private internals of the crate. The standard error! warn!,
  etc, macros now exclusively use the public API of the crate rather than "secret" internal APIs.
• Log::flush has been added to allow crates to tell the logging implementation to ensure that all
  "in flight" log events have been persisted. This can be used, for example, just before an
  application exits to ensure that asynchronous log sinks finish their work.

Removed

• The shutdown and shutdown_raw functions have been removed. Supporting shutdown significantly
  complicated the implementation and imposed a performance cost on each logging operation.
• The log_panics function and its associated nightly Cargo feature have been removed. Use the
  log-panics instead.

Changed

... (truncated)

Commits

• See full diff in compare view

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting @dependabot rebase.

---

Note: This repo was added to Dependabot recently, so you'll receive a maximum of 5 PRs for your first few update runs. Once an update run creates fewer than 5 PRs we'll remove that limit.

You can always request more updates by clicking Bump now in your Dependabot dashboard.

Dependabot commands and options

You can trigger Dependabot actions by commenting on this PR:

• @dependabot rebase will rebase this PR
• @dependabot ignore this [minor|major] version will close this PR and stop Dependabot creating any more for this minor/major version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this dependency will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)
• @dependabot use [this|these] label[s] will set the current labels as the default for future PRs for this repo and language

Additionally, you can set the following in your Dependabot dashboard:

• Update frequency (including time of day and day of week)
• Automerge options (never/patch/minor, and dev/runtime dependencies)
• Out-of-range updates (receive only lockfile updates, if desired)

Finally, you can contact us by mentioning @dependabot.