[ci skip]

Also fix some warnings to make it work.

Fixes #2198.

Changing the build config format causes cabal-install to repeatedly
rebuild the Cabal library during the configure->build->test cycle.

...and exploit new `cabal-install-1.18` package installing into `/opt/cabal/1.18/bin`

Should fix occasional DeterministicAr failures we're having.

See http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/
. Thanks to Mateusz Kowalczyk for the tip.

Hopefully.

A better solution would be to only allow failures in
TemplateHaskell/{profiling,dynamic} or sort out the dynamic/profiling libs
situation somehow.