[failing] Add specs for dirty context tag lists

Related #321.

Split tests for single-assertions
Call context-specific includes for each taggable_on call

Don't include Utils multiple times
Use modern .ruby-version filename

Get rid of .rvmrc in gitignore and use .ruby-version from now on.
Gives everyone freedom to choose RVM, rbenv and friends.
Create version 2.4.0.beta

I am releasing a beta version first, because the gem hasn't seen
updates in a while. It's just based on master. When nothing breaks
in a week, I will release 2.4.0 proper.
Modernize and cleanup gem plumbing

Moved version into the gem namespace
Modernized and updated the gemspec
Merge pull request #339 from shekibobo/dirty-context-tags

Fix dirty checking for subsequent calls to taggable_on. Fixes #321.
now tracking version 2.5.0
Update readme with up to date release planning

Also mentions compatibilities I want to see maintained.
These might still change though, depending on feedback from
the community.