Print an error if a `*_path` configuration is not a String

Fixes #278
Use the gem installation method for Raspbian

Raspberry Pi has ARM architecture, which is not supported by the Omnibus
Installer. But the gems should be installable for all current and future
versions, so we don't need to check the version number.
Merge pull request #300 from tmatilai/non-string-paths

Print an error if a `*_path` configuration is not a String
All the badges!
Trying out coveralls
Pin down ffi as v1.9.1 seems to break everything

Thanks to @allaire for the heads-up.
Mention knife-solo_data_bag gem in the docs. Fixes #83
Include information about berkshelf/librarian integration
Clarification around site-cookbooks
Take off pre1 so we can actually do source installs w/o bundler pathing
Update changelog
Finalize changelog
Post release bumps