Tweak Pango modules to work with 0.13.x packages

This involves some extra annotations here and there. The Cabal bump is
because all the 0.13.x packages require configCompilerEx from Cabal 1.18
onwards and the ‘vty’ package also requires 1.18. Fixes #576.

Conflicts:
	.travis.yml
	yi-contrib/yi-contrib.cabal
	yi/src/library/Yi/UI/Pango.hs
	yi/yi.cabal