Make the plugin Chef 11 compatible

* Removed unused require
* Allow any version between 10.14 that's smaller than 12
Added an env variable to set the Chef version
Initial .travis.yml file
Display chef version before launching specs
Using SSL rubygems + removing outdated auto-comment
Test more Chef versions
Merge pull request #6 from greenalto/chef_11

Make the plugin Chef 11 compatible
Merge pull request #8 from meatballhat/gemfile-brushup

Using SSL rubygems + removing outdated auto-comment