[BEAM-2642] Update Google Auth to 0.7.1

[lukecwik]

Google auth libraries 0.6.1 and 0.7.1 have the same versions of dependencies.

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This is a really annoying bug for Dataflow customers, if a 2.1.0 RC3 gets built. I would like to include this PR into it.

[lukecwik]

R: @jbonofre
CC: @tgroh

[coveralls]

Changes Unknown when pulling a98031c on lukecwik:beam2642release into ** on apache:release-2.1.0**.

[jbonofre]

LGTM, I'm merging.