Allow activesupport3 as well as activesupport4.

[jkingdon]

Either activesupport has what we need, and it seems unnecessary/bad to make
people upgrade from rails3 to rails4 just so they can run
transpec.

With this change, I was able to run transpec on a large rails 3.0 app and it has been working great.

[jkingdon]

Hmm, looking into this closer, if one runs transpec as bundle exec transpec instead of transpec, then one will get the error transpec is not part of the bundle. Add it to Gemfile. However, adding it to Gemfile seems to be the wrong solution, with the right one being just not using bundle exec.

So I guess this can be closed, unless there is sometimes a reason to add it to Gemfile.

[yujinakayama]

Yeah, basically transpec is intended to be run outside of the project's Bundler context. However, in some cases, it may be added to the Gemfile and should work properly as much as possible. So I'm 👍 for this.

[yujinakayama]

This should be '>= 3.0', '< 5.0'.

[yujinakayama]

OK, I've updated the dependency by myself. Thanks for the suggestion!

[jkingdon]

Sounds good. Sorry for not getting around to updating the pull request.