add gometaliner into travis build

[jimexist]

and gradually tighten the linting rules. for now unused and golint are left out.

some fixes are included in this PR.

using gometaliner instead of just golint because:

• it covers more cases, e.g. the typo checkings.
• it also allows for disabling rules w.r.t regex, e.g. when you don't want to enforce all the comment rules enforced by golint

• Part of Run lint (Python, Go) as a presubmit test #53

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This change is 

[k8s-ci-robot]

Hi @jimexist. Thanks for your PR.

I'm waiting for a kubernetes or tensorflow 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]

Coverage increased (+0.2%) to 37.963% when pulling b583ab7 on Jimexist:add-linter into 420f9aa on tensorflow:master.

[jimexist]

this fails for some reason...

[jimexist]

likely related:

• gotype regression alecthomas/gometalinter#358
• go/types: doesn't understands constants defined in cgo code golang/go#21712

[jlewi]

Can you add a TODO and link to appropriate issues?

[jimexist]

@jlewi done adding notes

[coveralls]

Coverage increased (+0.2%) to 37.963% when pulling 5d61049 on Jimexist:add-linter into 420f9aa on tensorflow:master.

[jlewi]

/ok-to-test

[jlewi]

nit: Could you update the PR description to explain why you chose gometalinter? As opposed to just using golint?

[jimexist]

@jlewi see PR description that I've updated

[coveralls]

Coverage increased (+0.2%) to 37.963% when pulling e2c44be on Jimexist:add-linter into 420f9aa on tensorflow:master.