This commit does two things:
1. Synchronizes `Gemfile.rails5.lock` with the latest changes in `Gemfile`
2. Adds a monkey patch for active record to avoid type casting in uniqueness
validator.
In Rails 5.0 there was introduced a bug [1] that causes to problems like
this:
```
1) API::Users POST /user/:id/gpg_keys/:key_id/revoke when authenticated revokes existing key
Failure/Error: let(:gpg_key) { create(:gpg_key, user: user) }
TypeError:
can't cast Hash
```
This bug was fixed in Rails 5.1 [2].
[1]: 811a4fa8eb
[2]: aa062318c4
Jan Provaznik