Allow QuickCheck 2.15

Author: tchoutri

servant-quickcheck/servant-quickcheck.cabal

@@ -49,7 +49,7 @@ library
     , mtl                    >=2.1    && <2.4
     , pretty                 >=1.1    && <1.2
     , process                >=1.2    && <1.7
-    , QuickCheck             >=2.7    && <2.15
+    , QuickCheck             >=2.7    && <2.16
     , servant                >=0.18.2 && <0.21
     , servant-client         >=0.17   && <0.21
     , servant-server         >=0.17   && <0.21