Remove hack for fixing Travis caching bug

The compiler: lines had the effect of getting us separate caches per GHC
version, but since travis-ci/travis-ci#4393 has been fixed we get separate
caches per matrix entry anyway.

.travis.yml

@@ -10,32 +10,21 @@ matrix:
   include:
     - os: osx
       env: CABALVER="1.24" GHCVER="8.0.1" TESTS="test_c"
-      compiler: ": #GHC 8.0.1"
     - env: CABALVER="1.20" GHCVER="7.6.3" TESTS="test_c"
-      # separate caches for different compiler versions
-      # until https://github.com/travis-ci/travis-ci/issues/4393 is resolved
-      compiler: ": #GHC 7.6.3"
       addons: {apt: {packages: [cabal-install-1.20,ghc-7.6.3,cppcheck,hscolour], sources: [hvr-ghc]}}
     - env: CABALVER="1.20" GHCVER="7.8.4" TESTS="test_c"
-      compiler: ": #GHC 7.8.4"
       addons: {apt: {packages: [cabal-install-1.20,ghc-7.8.4,cppcheck,hscolour], sources: [hvr-ghc]}}
     - env: CABALVER="1.22" GHCVER="7.10.3" TESTS="lib_doc doc"
-      compiler: ": #GHC 7.10.3"
       addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.3,cppcheck,hscolour], sources: [hvr-ghc]}}
     - env: CABALVER="1.22" GHCVER="7.10.3" TESTS="test_js"
-      compiler: ": #GHC 7.10.3"
       addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.3,cppcheck,hscolour], sources: [hvr-ghc]}}
     - env: CABALVER="1.22" GHCVER="7.10.3" TESTS="test_c"
-      compiler: ": #GHC 7.10.3"
       addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.3,cppcheck,hscolour], sources: [hvr-ghc]}}
     - env: CABALVER="1.24" GHCVER="8.0.1" TESTS="lib_doc doc"
-      compiler: ": #GHC 8.0.1"
       addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.1,cppcheck,hscolour], sources: [hvr-ghc]}}
     - env: CABALVER="1.24" GHCVER="8.0.1" TESTS="test_js"
-      compiler: ": #GHC 8.0.1"
       addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.1,cppcheck,hscolour], sources: [hvr-ghc]}}
     - env: CABALVER="1.24" GHCVER="8.0.1" TESTS="test_c"
-      compiler: ": #GHC 8.0.1"
       addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.1,cppcheck,hscolour], sources: [hvr-ghc]}}
 
 cache: