Revert "[ #2655 ] Temporally disabled make TAGS with GHC 7.8.4. (#2656)"

This reverts commit 48ee5fd.

See haskell/cabal#4624 (comment).

Author: asr

.travis.yml

@@ -362,8 +362,7 @@ script:
        make BUILD_DIR=$BUILD_DIR lib-interaction;
     fi
 
-# ASR (2017-07-25). Temporally disabled for GHC 7.8.4 [Issue #2655].
-  - if [[ $TEST = "MAIN" && $GHC_VER != "7.8.4" ]]; then
+  - if [[ $TEST = "MAIN" ]]; then
        make BUILD_DIR=$BUILD_DIR TAGS;
     fi