Fix auth token revocation (broken since 0.6.3)

[tadast]

Looks like https://github.com/google/google-auth-library-ruby/pull/147/files has changed the auth revocation URL from https://accounts.google.com/o/oauth2/revoke to https://oauth2.googleapis.com/revoke and the new URL seems to respond with a 404. This is because /revoke should be invoked with a POST.

Update The HTTP method for /revoke to be a POST.

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[coveralls]

Coverage remained the same at 98.624% when pulling 3913377 on tadast:fix-token-revocation into 9b49da3 on google:master.