.travis.yml: recache the user package DB after installing GHC.
.travis.yml: recache the user package DB after installing GHC.

Should fix occasional DeterministicAr failures we're having.

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