Update rspec-rails from 3.3.3 to 3.4.2.

This allows the removal of the monkey patch from this commit: 47ff1c56

It'll also make it slightly easier to upgrade to 3.5.0 later.

Changelog: https://github.com/rspec/rspec-rails/blob/master/Changelog.md#340--2015-11-11

See merge request !3999