Migrate Loop cop to Parser 2.0.0.beta8

Now Parser generates `while_post` and `until_post` nodes for
`begin...end while/until` source.
So we just need to catch them.
https://github.com/whitequark/parser/commit/679b526
Migrate BlockNesting cop to Parser 2.0.0.beta8

We need to catch the newly added `while_post` and `until_post` nodes.
Migrate RescueModifier cop to Parser 2.0.0.beta8

Parser now generates `kwbegin` node for explicit `begin end` source
to differentiate it with implicit begin.
https://github.com/whitequark/parser/issues/70#issuecomment-19525402
Revert "Mark a spec as broken with Parser 2.0.0.beta6"

This reverts commit 71e4067214f60f917509cd0362d1d60dce37e91e.
Update Parser dependency to 2.0.0.beta8
Merge pull request #302 from yujinakayama/migrate-to-parser-2.0.0.beta8

Migrate to Parser 2.0.0.beta8