Remove version requirements from guard plugin gems

Let bundle manages dependencies.
Bump guard to 2.10.2

Removed default plugin scope. At first, there was no specs for this
library, so we disabled it temporarily. But with guard 2.10.2, this is
invalid statement and we added some specs, so now we remove plugin
scope.

See bafa69da3748931863622f1d5953b5d2ca403d50.
Merge pull request #136 from yous/bump-guard

Bump guard