Refactor Gemfile / update dependecies

[mnquintana]

Our Gemfile currently references very few version numbers, and this is a problem because anytime someone inadvertently updates Gemfile.lock (like in #535) we inadvertently upgrade Reservations' dependencies.

We should use the appropriate version identifiers (see http://bundler.io/v1.6/gemfile.html) - for example:

gem 'capybara', '~> 2.0.0'

We only want to automatically upgrade to service releases for capybara, ie. versions between 2.0.0 and 2.1.

[squidgetx]

I think this can wait until 4.0.0, at which point we should just go on a massive updating spree. FontAwesome 4, cancancan, Spring, the works. After that we can then go and specify version numbers for everything