Use go1.12 and go modules

[s4nji]

• Remove Gopkg.toml and Gopkg.lock
• Add go.mod and go.sum
• Update Dockerfile to use go1.12
• Update Travis to use go modules

This is done mainly to resolve vulnerabilities that affects Janus which are fixed on go 1.12.8

[codecov[bot]]

Codecov Report

Merging #411 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #411   +/-   ##
=======================================
  Coverage   40.48%   40.48%           
=======================================
  Files          76       76           
  Lines        2655     2655           
=======================================
  Hits         1075     1075           
  Misses       1517     1517           
  Partials       63       63

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[RTodorov]

+1

[RTodorov]

+1