Demonstrate that call to bundle update fails in multi-platform spec

For this spec to make sense, the call should succeed, since otherwise
the cache update never runs and the spec will pass trivially.
Ensure Gemspec.lock and Gemfile match in multi-platform cache spec

The activerecord 2.3.3 gem needs activesupport 2.3.2, which was not
included in the Gemfile.lock or cache. This caused bundle update to
fail. This change simplifies the set of needed gems, thereby returning
to an older version of this spec. By adding the mri platforms to the
block for 'rack' we keep the check for allowed platforms.
Make describe block description match tested method
Move test of valid platforms out of multi-platform spec
Use Ruby 1.8-compatible hash syntax
Merge pull request #3034 from mvz/ensure-success

Ensure `bundle update` runs succesfully in specs of multi-platform cache behavior