Maintenance: Fix cabal install into Travis cache

Author: HeinrichApfelmus

.travis.yml

@@ -58,7 +58,7 @@ script:
     cd dist
     tar zxfv "$SRC_TGZ"
     cd "$PKGVER"
-    (cabal configure --enable-tests && cabal build && cabal test && cabal install) || exit 1
+    (cabal configure --enable-tests && cabal build && cabal test && cabal install --force-reinstalls) || exit 1
     cd $ORIGDIR
   done
   set +ex