Drop broken Haskell compilers.

 - ghc versions 6.10.4, 6.12.3, and 7.2.2 are broken, and 6.10.2-binary is no
   longer necessary after those versions have been dropped

 - halvm version 2.4.0 hasn't compiled in a long time

 - uhc version 1.1.9.4 hasn't compiled in a long time