Add dummy rails app to make tests run

[ritxi]

Hi, I've just fixed test suite to run all tests without missing dependendies errors

[janko]

Thanks for the pull request. Unfortunately, these changes tie shrine-reform to Rails (and ActiveModel validations), which is not ideal for people using Reform outside of Rails (and with dry-validation). Though shrine-reform doesn't seem to actually work with dry-validation anyway (see #2) 🤷‍♂

Note that @apotonick is working on Reform 3.0, which will be a rewrite of the existing API. But that will perhaps take time, so maybe it makes sense to make shrine-reform work with Reform 2.x. I've been holding off on updating compatibility until Reform 2.3 is released, however, it seems that will be delayed indefinitely in favour of version 3.0.