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.

.travis.yml

@@ -1,9 +1,12 @@
 language: ruby
 before_script:
+	- rake setup
   - gem install rake -v "~> 0.9.2.2"
   - gem install minitest -v "~> 3.2"
 rvm:
   - 1.8.7
   - 1.9.3
   - 2.0.0
-
+env:
+	- USE_RUBYGEMS="1.8.25" UPTO_RUBY="2.0.0"
+	- USE_RUBYGEMS="2.0.3"

rakefile.rb

@@ -41,13 +41,14 @@
   lib_dirs = ["lib", "test"].join(File::PATH_SEPARATOR)
   test_files = FileList["test/**/test*.rb"].gsub("test/", "")
 
-  if ENV["TRAVIS"] && ENV["CI"] && RUBY_VERSION < "2.0.0"
-    %w(1.8.25 2.0.3).each do |version|
-      sh "gem update --system #{version}"
-      ruby "-I#{lib_dirs} -e \"ARGV.each { |f| require f }\" #{test_files}"
-    end
-  else
-    ruby "-I#{lib_dirs} -e \"ARGV.each { |f| require f }\" #{test_files}"
+  ruby "-I#{lib_dirs} -e \"ARGV.each { |f| require f }\" #{test_files}"
+end
+
+desc "Sets up the test environment"
+task :setup do
+  if ENV["USE_RUBYGEMS"] && (!ENV["UPTO_RUBY"] || RUBY_VERSION < "#{ENV["UPTO_RUBY"]}")
+    sh "gem update -q --system #{ENV["USE_RUBYGEMS"]}"
+    puts "Using RubyGems #{`gem --version`}"
   end
 end