zeal: pin at Qt 5.5

The package requires Qt Webkit, which is removed from Qt 5.6.

pkgs/top-level/all-packages.nix

···
 
   xlsx2csv = pythonPackages.xlsx2csv;
 
-  zeal = qt5.callPackage ../data/documentation/zeal { };
+  zeal = qt55.callPackage ../data/documentation/zeal { };
 
 
   ### APPLICATIONS