Clean up specs located in spec/bundler

Out of curiosity I started to look how `bundler` specs are written and I found several places which can be refactored/improved just a little.

Here is a list what has been changed:
- use `subject` and `let` instead of instance variables
- use `except` instead of `should`
- remove unused variables
- test exception messages
- improve tests descriptions
- simplify and improve some test scenarios (especialy in `spec/bundler/gem_helper_spec.rb`)