update transformers version to 0.6

Author: epoberezkin

.gitignore

@@ -2,3 +2,4 @@ dist/
 .cabal-sandbox
 cabal.sandbox.config
 .stack*
+dist-newstyle/

network-transport.cabal

@@ -68,7 +68,7 @@ Library
                    binary >= 0.5 && < 0.9,
                    bytestring >= 0.9 && < 0.12,
                    hashable >= 1.2.0.5 && < 1.5,
-                   transformers >= 0.2 && < 0.6,
+                   transformers >= 0.6 && < 0.7,
                    deepseq >= 1.0 && < 1.5
   if impl(ghc < 7.6)
     Build-Depends: ghc-prim >= 0.2 && < 0.4