Allow singleton-bool 0.1.6

Builds fine and all tests pass.

Author: felixonmars

servant/servant.cabal

@@ -90,7 +90,7 @@ library
   -- i.e. re-export, or allow using without direct dependency
   build-depends:
       http-api-data          >= 0.4.1    && < 0.4.3
-    , singleton-bool         >= 0.1.4    && < 0.1.6
+    , singleton-bool         >= 0.1.4    && < 0.1.7
 
   -- Other dependencies: Lower bound around what is in the latest Stackage LTS.
   -- Here can be exceptions if we really need features from the newer versions.