Bump guard

- Remove version requirements from `guard-rspec` and `guard-rubocop`. Let bundle manages their 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.