gpython: use peterh/liner@v1.1.0

[sbinet]

This CL bumps go.mod to use peterh/liner@v1.1.0.

Also really test Go modules by enabling GO111MODULE=on, only for master.

[codecov-io]

Codecov Report

Merging #18 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #18 +/- ##
=======================================
Coverage 64.52% 64.52% =======================================
Files 55 55 Lines 9746 9746 =======================================
Hits 6289 6289 Misses 3010 3010 Partials 447 447

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[corona10]

@sbinet Can you squash all your commits into the single commit?

[sbinet]

I could but these 2 commits really do 2 different things. related, but different.
I can send 2 PRs if you'd prefer.