Revert Gem::Ext::BuildError change

I tested this in `irb`, but when I ran Command-T, it dies with:

  NameError: uninitialized constant Gem::Ext

So will have to try again.

autoload/commandt.vim

@@ -167,7 +167,7 @@ ruby << EOF
     else
       $command_t = CommandT::Stub.new
     end
-  rescue Gem::Ext::BuildError, LoadError
+  rescue LoadError
     load_path_modified = false
     ::VIM::evaluate('&runtimepath').to_s.split(',').each do |path|
       lib = "#{path}/ruby"