Allow swagger2 2.6

Builds fine and all tests pass.

Author: felixonmars

servant-swagger.cabal

@@ -84,7 +84,7 @@ library
                      , lens                      >=4.17     && <4.19
                      , servant                   >=0.17     && <0.18
                      , singleton-bool            >=0.1.4    && <0.2
-                     , swagger2                  >=2.3.0.1  && <2.6
+                     , swagger2                  >=2.3.0.1  && <2.7
                      , text                      >=1.2.3.0  && <1.3
                      , unordered-containers      >=0.2.9.0  && <0.3