Use https for rubygems.org
Remove vagrant gem from dependencies

Prefer the Vagrant installer.
Remove Berkshelf from local tests

The current Berkshelf version depends on Chef ~> 10.16.2 and older
have incopatible requirements for common dependencies. So drop Berkshelf
for now and run knife cookbook test directly in the cookbook root.

Vagrant still uses Berkshelf.
Add description to the default Rake task
Explicitly declare Travis to test with Chef 10 and 11
Add syntax highlighting for code blocks in readme