Build Haddock documentation on Travis.
Build Haddock documentation on Travis.

Fixes #2198.

sopvop/cabal,beni55/cabal,corngood/cabal,edsko/cabal,thoughtpolice/cabal,gridaphobe/cabal,Heather/cabal,gbaz/cabal,themoritz/cabal,headprogrammingczar/cabal,fugyk/cabal,bennofs/cabal,Peaker/cabal,valderman/cabal,headprogrammingczar/cabal,Helkafen/cabal,randen/cabal,enolan/cabal,mightybyte/cabal,plumlife/cabal,mightybyte/cabal,garetxe/cabal,themoritz/cabal,Helkafen/cabal,christiaanb/cabal,ian-ross/cabal,kosmikus/cabal,thoughtpolice/cabal,typelead/epm,ian-ross/cabal,mydaum/cabal,ghc-android/cabal,edsko/cabal,eric-stanley/cabal,garetxe/cabal,themoritz/cabal,mydaum/cabal,ghc-android/cabal,trskop/cabal,thomie/cabal,randen/cabal,Peaker/cabal,kolmodin/cabal,gbaz/cabal,trskop/cabal,fugyk/cabal,anton-dessiatov/cabal,valderman/cabal,seereason/cabal,anton-dessiatov/cabal,kosmikus/cabal,sopvop/cabal,beni55/cabal,kolmodin/cabal,rimmington/cabal,mmakowski/cabal,thomie/cabal,x-y-z/cabal,gridaphobe/cabal,corngood/cabal,Heather/cabal,triplepointfive/cabal,plow-technologies/cabal,lukexi/cabal,sopvop/cabal,plumlife/cabal,triplepointfive/cabal,martinvlk/cabal,seereason/cabal,plow-technologies/cabal,typelead/epm,bennofs/cabal,eric-stanley/cabal,enolan/cabal,mmakowski/cabal,martinvlk/cabal,rimmington/cabal,x-y-z/cabal,christiaanb/cabal,lukexi/cabal