shoulda-context is incompatible with shoulda-matchers >= 1.5

[This commit](https://github.com/thoughtbot/shoulda-matchers/commit/4f8684c6dbad2837a1860fae74d88b7d64f0f1e2) broke `assert_accepts` and `assert_rejects`. Since [shoulda.gemspec allows for matchers >= 1.4.1](https://github.com/thoughtbot/shoulda/blob/master/shoulda.gemspec#L22) `gem install shoulda` will be broken :crying_cat_face: