Added support for testing multiple RubyGems
Adds a :setup task to rakefile.rb that uses environment variables to
determine which version of RubyGems the tests should be run against.

    USE_RUBYGEMS        - Specifies what version of RubyGems to test
                          against. (e.g. USE_RUBYGEMS="1.8.12")
    UPTO_RUBY           - Specifies the cutoff version of Ruby so
                          that :setup won't try to downgrade RubyGems
			  on Ruby >= 2.0

Modified the .travis.yml file to include the environment variables to test
against.
Fix formatting in .travis.yml
Modified .travis.yml matrix
Modified the .travis.yml matrix to only include USE_RUBYGEMS for Rubies <
2.0.
Modified .travis.yml for environment config
Fixed .travis.yml to exclude 2.0.0/1.8.25
:test task now reports Ruby/RubyGems versions
Removed UPTO_RUBY as it is no longer required