Bump version requirements to base>=4.6, Cabal>=1.20

Cabal>=1.20 allows control over parallelism when compiling grammars from
Setup.hs and WebSetup.hs.

base>=4.6 allows conditional compilation with CPP to be eliminated from
a few modules.

base-4.6 corresponds to GHC 7.6.3, which is what you get in
Debian 8 (aka jessie, aka oldstable) from 2015.

WebSetup.hs

@@ -98,14 +98,13 @@ execute command args =
     showArg arg = if ' ' `elem` arg then "'" ++ arg ++ "'" else arg
 
 -- | This function is used to enable parallel compilation of the RGL and
--- example grammars, but it is commented out by default
--- to avoid casing problems for developers using Cabal<1.20
+-- example grammars
 numJobs flags =
     if null n
     then ["-j","+RTS","-A20M","-N","-RTS"]
     else ["-j="++n,"+RTS","-A20M","-N"++n,"-RTS"]
   where
     -- buildNumJobs is only available in Cabal>=1.20
-    n = {-case buildNumJobs flags of
+    n = case buildNumJobs flags of
           Flag mn | mn/=Just 1-> maybe "" show mn
-          _ ->-} ""
+          _ -> ""