Switch rails-rspec src-dir

[lackita]

Rails puts most of its source files under app rather than lib. This
updates the src-dir to allow easy switching between source and test
files in this case.

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Thanks!

[bbatsov]

Makes sense. I guess we should do the same for rails-minitest as well.

[lackita]

Good call, I'll try to change that tomorrow

[lackita]

Alright, I think that's all set now. Let me know if you want me to change anything else.

[bbatsov]

Thanks!