Allow Travis failures for now.

A better solution would be to only allow failures in
TemplateHaskell/{profiling,dynamic} or sort out the dynamic/profiling libs
situation somehow.
TODO.
Enable GHC HEAD builds on Travis CI.
Allow 'PROG-options' in cabal.config.
Turn on '-fno-ignore-asserts' in 'cabal.config'.

Developers should use builds with assertions enabled. See #1619.