base64 encode version token

[jiayuanmasc]

Background
#16 (comment)

Changes

• base64 encode versionToken as specified by the api docs

[googlebot]

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[thatjiaozi]

Hi,
Please sign the CLA before we can proceed, thanks!

[jiayuanmasc]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[thatjiaozi]

Thansk for working on this