FR: Support Go 1.6

[tbpg]

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To be compatible with all supported versions on AppEngine (and therefore be able to use in examples in https://github.com/GoogleCloudPlatform/golang-samples), it would be great to support Go 1.6. See previous discussion in #110.

Relevant Code:

Code using import "context", request.WithContext, subtests, etc. will need to be updated.

PR coming up.

[tbpg]

With #120 merged, the 1.6.x build is now green. Closing this.