Travis fails for GHCVER=head

[MathiasBartl]

Maybe add:
matrix:
allow_failures:

• env: GHCVER=head
  to travis file as recommended in
  https://github.com/hvr/multi-ghc-travis

[MathiasBartl]

Ok, apparently travis couldn't just find GHC.
https://travis-ci.org/MathiasBartl/Concurrent_Datastructures/jobs/32718347