Remove or clarify dependency on tus-server

[jonleighton]

This plugin defines a dependency on tus-server (a Ruby implementation of a Tus server) in the gemspec. However, it never actually requires any files from that gem.

That said, it does implicitly depend on tus-server in this line:

shrine-tus/lib/shrine/storage/tus.rb

Line 20 in d586ee0

rescue ::Tus::NotFound

That constant is defined in tus-server here.

Therefore, if I use shrine-tus without explicitly requiring tus-server, then I'll get a NameError if I hit an error in this method.

There are two potential solutions:

1. Remove the dependency on tus-server. This would be my preference - I don't think shrine-tus as a Tus client should care about what server implementation is used. (On my project, we're using tusd, therefore we don't want/need the tus-server gem at all.)
2. Require tus/errors so that the constant gets defined.