Fixes for compatibility with Go 1.16

[cinchurge]

Ref https://golang.org/doc/go1.16#go-command

go modules are now enabled by default, so go get would fail
without explicitly turning it off via GO111MODULE=off

additionally, go build -i now returns an error during tests. since
that flag has been deprecated, remove it so the tests can pass.

[codecov]

Codecov Report

Merging #821 (59254b6) into dev (61d863a) will increase coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #821      +/-   ##
==========================================
+ Coverage   87.49%   87.79%   +0.29%     
==========================================
  Files          42       42              
  Lines        3702     3702              
==========================================
+ Hits         3239     3250      +11     
+ Misses        344      333      -11     
  Partials      119      119              

Impacted Files	Coverage Δ
preinit_connection.go	92.12% <0.00%> (-1.58%)	⬇️
peer.go	93.47% <0.00%> (-0.44%)	⬇️
connection.go	87.56% <0.00%> (+3.70%)	⬆️

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[prashantv]

Can we also update .travis.yml to test against stable and oldstable?