v0.18.0

Update Go to 1.15 and module dependencies.
Also switches from Travis to GitHub Actions for CI and adds a build for arm32.

SHA-256 checksums:

fae705c317038b91f704d1a35fb2c6c7c0669cad2573b48018a29878bffcc5df  confd-0.18.0-darwin-amd64
29552bc6ca2cfe29530e693af197d5652104963b496145b2e5e30aa577bcd7a6  confd-0.18.0-linux-amd64
e1ddf022cc6b5df6ee3dabc5eab692e7abe1ef8d1aa9291dde7126a950f10943  confd-0.18.0-linux-arm32
52adf339333a10c23041395b6f2b130e2722d7a8c18ad8a16013705cf679dcd2  confd-0.18.0-linux-arm64
3572ac6484cf9a412f0bf32a6179d8b93fb1cf1bcd052a19e6fb5767bed60342  confd-0.18.0-windows-amd64.exe