Interface - v1.3.0

[jessedoyle]

• Break out Prawn::Icon::Interface into its own file [#27].
• Add simplecov as a development dependency and require 100% coverage.
• Update rubocop to 0.44.1.
• Bump version to 1.3.0.

[jessedoyle]

Had to remove the simplecov 100% restriction! For some reason, JRuby reports no coverage data.