Update Travis to run Go 1.7 and 1.8 versions

[cristaloleg]

Before opening your pull request, please make sure that you've:

• signed Uber's Contributor License Agreement;
• added tests to cover your changes;
• run the test suite locally (make test); and finally,
• run the linters locally (make lint).

Thanks for your contribution!

There is also an option for Travis called 'allow_failures'.
Am I need to move to update PR to something like this?

matrix:
  allow_failures:
    - go: tip
    - go: 1.5
    - go: 1.6

(I've checked last 3 checkboxes in advance)

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ cristaloleg
❌ Oleg Kovalov

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[cristaloleg]

I did a mistake with email in commit. Will create another PR. I'm sorry..