Increase upper bound on containers (#328)

Fixes #326

Author: Gabriella439

turtle.cabal

@@ -54,7 +54,7 @@ Library
         async                >= 2.0.0.0 && < 2.3 ,
         bytestring           >= 0.9.1.8 && < 0.11,
         clock                >= 0.4.1.2 && < 0.8 ,
-        containers           >= 0.5.0.0 && < 0.6 ,
+        containers           >= 0.5.0.0 && < 0.7 ,
         directory            >= 1.0.7   && < 1.4 ,
         exceptions           >= 0.4     && < 0.11,
         foldl                >= 1.1     && < 1.5 ,