rubygems: fix variable name

* lib/rubygems/compatibility.rb: fix out-of-scope local variable.

* lib/rubygems/specification.rb (Gem::Specification#validate): get
  rid of conflict between a local variable and a method name.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48985 b2dd03c8-39d4-4d8f-98ff-823fe69b080e