Add golint to ci

[cappyzawa]

I enable to run golint in ci.
And I fixed based on golint.
Before: Job #2330.1 - labstack/echo - Travis CI
After: Job #2331.1 - labstack/echo - Travis CI

[codecov]

Codecov Report

Merging #1243 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1243   +/-   ##
=======================================
  Coverage   81.57%   81.57%           
=======================================
  Files          26       26           
  Lines        1927     1927           
=======================================
  Hits         1572     1572           
  Misses        248      248           
  Partials      107      107

Impacted Files	Coverage Δ
echo.go	87.63% <ø> (ø)	⬆️

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