Changed import in access_token.go

[liannarnold]

I was unable to run go get github.com/sfreiberg/gotwilio due to this error access_token.go:91:7: undefined: jwt.StandardClaims. I was able to fix it on my machine by changing the import in access_token.go back to github.com/dgrijalva/jwt-go and running go get github.com/dgrijalva/jwt-go.