Updating Gemfile

Hello,

This patch makes bundler use https instead of http for installing gems.

It also removes this warning when you do a bundle install:

````
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
Fetching gem metadata from http://rubygems.org/.........
Fetching gem metadata from http://rubygems.org/..
````