fix: add go module support

[appleboy]

fix #171

See go module testing: http://drone.wu-boy.com/appleboy/gin-jwt/190/7

[codecov-io]

Codecov Report

Merging #180 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #180   +/-   ##
=======================================
  Coverage   96.59%   96.59%           
=======================================
  Files           1        1           
  Lines         352      352           
=======================================
  Hits          340      340           
  Misses          8        8           
  Partials        4        4

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