Use bundle exec

Sometimes we can get the following errors

/Users/username/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/bundler-1.3.5/lib/bundler/runtime.rb:33:in `block in setup': 
You have already activated rspec-core 2.14.4, but your Gemfile requires rspec-core 2.13.1. 
Using bundle exec may solve this. (Gem::LoadError)

Bundle exec solves such issues