Compare RUBY_VERSION as string not by #to_f

- before: RUBY_VERSION.to_f >= 2.0
- after: RUBY_VERSION >= '2.0'

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2554 f4f36d40-9719-0410-9868-dde021a23eda
Fix wrong assertion in test_upto

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2555 f4f36d40-9719-0410-9868-dde021a23eda