Removed explicit include of RR adapter

As per RR's deprecation warning:
RR now has an autohook system. You don't need to  in your test framework's base class anymore.
Add version guard for pre 1.9.3 Rubies
Use RUBY_VERSION constant instead of bundler's :platform to catch 1.8.7 and 1.9.2
Add version guard for shoulda-matchers

The shoulda-matchers gem requires Ruby 1.9.2 or above from v2.6.0
Corrected version of shoulda-matchers for Ruby < 1.9.2
Merge branch 'gem-version-guards-for-old-rubies' into ruby-2.1-support

Added changeset containing version guards for gems that have added
requirements for specific Ruby versions.