Fix build failure on go version 1.12

[joseph-v]

This PR fixes issue #20
The default value of GO111MODULE is different in go versions (GO111MODULE=auto in go 1.12, but GO111MODULE=on in go 1.13)
https://blog.golang.org/modules2019

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind new feature
/kind bug fix
/kind cleanup
/kind revert change
/kind design
/kind documentation
/kind enhancement

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Test Report Added?:

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:
/kind TESTED
/kind NOT-TESTED

Test Report:

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #21   +/-   ##
=======================================
  Coverage   41.40%   41.40%           
=======================================
  Files          33       33           
  Lines        5598     5598           
=======================================
  Hits         2318     2318           
  Misses       2819     2819           
  Partials      461      461           

[kumarashit]

LGTM

[skdwriting]

LGTM