Add Rails 4 support

@jeromecornet @kmcphillips Can you take a look at this?

In attempting to use this on a Rails 4 app, I only found three problems:

1. A hard dependency on Rails ~3.1 in the gemspec. I changed this to use `>= 3.1` instead.

2. Use of `match` in the routes file, which is no longer supported. I changed these references to use `get` instead.

2. The format of the default `Gemfile` has changed. The `add_bootstrap_gem` method relied on comments to work properly, and those comments changed. I changed the method to properly use the `gem_group` command, which is a more correct way of doing things anyway.