Use Parser 2.1

FWIW, `SpaceAroundBlockBraces` reports [false positives](https://travis-ci.org/yujinakayama/transpec/builds/15960833) with a combination of Ruby 1.9 (including JRuby in 1.9 mode), RuboCop 0.15 and Parser 2.1, though it seems to be fixed on the current `master`.