Replace IdentityHandler's parameter jwt.MapClaims with MapClaims

Hi, 

I noticed type `MapClaims` of package `"gopkg.in/dgrijalva/jwt-go.v3"`  is used by `GinJWTMiddleware.IdentityHandler`'s parameter, which expose the dependency of  `gopkg.in/dgrijalva/jwt-go.v3`.

I think it would be cleaner that if we replace the dependency with `"github.com/appleboy/gin-jwt"`'s `MapClaims`.