2.1.0 - FontAwesome 4 Shim

Add a compatibilty class that includes FontAwesome 4 shim data from the FontAwesome metadata.
Effectively rewrite all instances of fa-* to their appropriate FontAwesome 5 font family.
Perform the icon key rewrite at the higest scope possible in Prawn::Icon::Interface to retain backwards compatibility.
Add a deprecation warning whenever fa-* references are encountered.
Disable the Layout/IndentHeredoc rubocop rule to not have to include further dependencies for heredoc indentation.
Bump version to 2.1.0.

See the discussion here.

jessedoyle / prawn-icon