Force Token URL to `http://metadata.google.internal/...`

In certain environments (such as inside a docker container) 'metadata' does not resolve.
Forcing URL to `http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token` fix this.

See https://developers.google.com/compute/docs/metadata#metadataserver

Closes #28.
Closes #30.