Maybe you need to give the full path with sudo...

.travis.yml

@@ -30,7 +30,7 @@ install:
  - echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
  - |
    if [ "$GHCVER" = "7.2.1" ]; then
-     sudo ghc-pkg trust base # To avoid a SafeHaskell bug on GHC 7.2.1
+     sudo /opt/ghc/$GHCVER/bin/ghc-pkg trust base # To avoid a SafeHaskell bug on GHC 7.2.1
    fi
  - travis_retry cabal update
  - cabal install --only-dependencies