Bring Go to 1.15.6 in Travis

[gliptak]

No description provided.

[k8s-ci-robot]

Hi @gliptak. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[coveralls]

Pull Request Test Coverage Report for Build 1364

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 81.713%

---

Totals
Change from base Build 1359:	0.0%
Covered Lines:	1622
Relevant Lines:	1985

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💛 - Coveralls

[gliptak]

/assign @jsafrane

[wongma7]

/ok-to-test
/lgtm
/approve
thanks

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gliptak, wongma7

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