travis-ci: install many Haskell dependencies via apt-get instead of building them locally
test.hs: the cabal build places the compiled git-annex binary into a 'dist/build/git-annex' directory

Tests must be run having that directory in $PATH, otherwise git won't find it.
git-annex.cabal: relax 'mtl >= 2.1' constraint to 'mtl >= 2'

Git-annex doesn't actually depend on mtl version 2.1. By relaxing this
constraint, builds can succeed on Ubuntu 12.04.