merger.rb: suppress warnings

* tool/merger.rb (version_up, tag, default_merge_branch): suppress
  syntax warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39424 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merger.rb: runnable

* tool/merger.rb: make runnable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39425 b2dd03c8-39d4-4d8f-98ff-823fe69b080e