Upgrade parser gem dependency to 2.0.0.pre5

This branch upgrades parser to 2.0.0.pre5.

During the process bug https://github.com/whitequark/parser/issues/98 was found in parser, and the fix https://github.com/whitequark/parser/pull/99 was provided to fix it.

In the meantime this code works-around the issue by bypassing helper method `Parser::Source::Comment.associate` and uses `Parser::Source::Comment::Associator` directly.

It's up to you on whether or not you merge this in as-is, or if you wait until the next parser release. I will be happy to update this PR with the new version and revert the work-around if you decide to wait.