Revert "Give up support for ghc 7.4."

This reverts commit e0c83f74f3.

.travis.yml

@@ -13,6 +13,9 @@ before_cache:
 
 matrix:
   include:
+    - env: CABALVER=1.16 GHCVER=7.4.2 GHCOPTS=-Werror
+      compiler: ": #GHC 7.4.2"
+      addons: {apt: {packages: [cabal-install-1.16,ghc-7.4.2], sources: [hvr-ghc]}}
     - env: CABALVER=1.16 GHCVER=7.6.3 GHCOPTS=-Werror
       compiler: ": #GHC 7.6.3"
       addons: {apt: {packages: [cabal-install-1.16,ghc-7.6.3], sources: [hvr-ghc]}}

make_travis_yml.hs

@@ -90,7 +90,7 @@ genTravisFromCabalFile fn xpkgs = do
         let cvs = disp' (lookupCabVer gv)
             gvs = disp' gv
             ghcopts = if gv >= Version [7,10,0] []
-                         then "-Werror"
+                         then ""
                          else "-Werror"
 
             xpkgs' = concatMap (',':) xpkgs

pandoc.cabal

@@ -11,7 +11,7 @@ Bug-Reports:     https://github.com/jgm/pandoc/issues
 Stability:       alpha
 Homepage:        http://pandoc.org
 Category:        Text
-Tested-With:     GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.2
+Tested-With:     GHC == 7.4.2, GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.2
 Synopsis:        Conversion between markup formats
 Description:     Pandoc is a Haskell library for converting from one markup
                  format to another, and a command-line tool that uses