Silence RSpec's deprecation warnings.

RSpec deprecated `mock` in favour of `double` [a year ago](/rspec/rspec-mocks/commit/843a40f). In addition to that, `let` and `subject` declarations are not intended to be called in a `before(:all)` hook. This is deprecated behavior that will not be supported in RSpec 3.

Version 2.6 as specified in the Gemfile is three years old.