update test support:

* add `chefspec` coverage
* add CI runs for both chef 10 and 11
* add CI runs for ruby 2.1
Merge pull request #66 from phlipper/test-coverage

Update test support
adds support for customizable apt sources
Merge pull request #69 from engineyard-cookbooks/ppa_support

adds support for customizable apt sources
add contribution from @alenia to the README