Make bound for safe dependent in ghc version.

safe 0.3.10 is having issues with cabal 1.20 and ghc 7.6.3, the
hvr ghc ppa doesn't provide a new enough cabal library. Temporary
fix until hvr ghc ppa is fixed or we deprecate support for 7.6.3.

idris.cabal

@@ -289,7 +289,6 @@ Library
                 , vector < 0.12
                 , vector-binary-instances < 0.3
                 , zip-archive > 0.2.3.5 && < 0.4
-                , safe >= 0.3.9
                 , fsnotify >= 0.2 && < 2.2
                 , async < 2.2
 
@@ -302,6 +301,14 @@ Library
   else
      build-depends: process < 1.5
 
+  -- safe 0.3.10 is having issues with cabal 1.20 and ghc 7.6.3, the
+  -- hvr ghc ppa doesn't provide a new enough cabal library. Temporary
+  -- fix until hvr ghc ppa is fixed or we deprecate support for 7.6.3.
+  if impl(ghc < 7.8.4)
+     build-depends: safe == 0.3.9
+  else
+     build-depends: safe >= 0.3.9
+
   Extensions:     MultiParamTypeClasses
                 , DeriveFoldable
                 , DeriveTraversable