Use Parser 2.1 or later
Revert "[Fix #647] Workaround for Ruby 2.1"

This reverts commit 657e91110c29c69b29f0db027289f3091651cfac.
Handle API change of Parser::Diagnostic.new

https://github.com/whitequark/parser/commit/8cb572c2872edd6ee89343cea3da7ad691543c32
Test against Ruby 2.1.0-rc1 on Travis CI
Merge pull request #696 from yujinakayama/parser-2.1

Use Parser 2.1