Fix lint and coverage

[yurishkuro]

Problem

• The old go get -u github.com/golang/lint/... no longer works
• The new go get -u golang.org/x/lint/golint/... does not work in older Go versions (it uses type alias)
• Coverage was never uploaded

Fixes

• Use Travis build matrix with env vars to control LINT and COVERAGE
• Only run lint and coverage on tip, not on older versions

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6b6f234). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #202   +/-   ##
=========================================
  Coverage          ?   58.74%           
=========================================
  Files             ?       14           
  Lines             ?      829           
  Branches          ?        0           
=========================================
  Hits              ?      487           
  Misses            ?      302           
  Partials          ?       40

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[yurishkuro]

these should go to user's global gitignore