Rename umbImageGravity.directive.js to umbimagegravity.directive.js

Changed file name to be consistent with the rest of our file names

src/Umbraco.Web.UI.Client/src/common/directives/imaging/umbimagegravity.directive.js

@@ -154,4 +154,4 @@ angular.module("umbraco.directives")
 
 				}
 			};
-		});
+		});