Not sure why the GHC 8 build needs this

hamishmack · hamishmack · commit 70fa72116d6f · 2016-12-07T16:33:35.000+13:00
Strange that the GHC 7.10 travis build seems to work ok without it
diff --git a/travis/build.sh b/travis/build.sh
@@ -8,6 +8,7 @@ if [[ -d .cabal && -d .ghc ]]; then
 fi
 
 cabal update
+cabal install gtk2hs-buildtools
 
 cabal new-build
 
