Bump CI tested go versions

[tmthrgd]

This bumps Travis CI to the last two supported golang releases, as documented in the readme: go1.14 and go1.15.

It also bumps the GitHub Code Action CI to go1.14 as it doesn't yet supported go1.15 (see actions/setup-go#74).

This is blocking #1155.

[codecov-io]

Codecov Report

Merging #1172 into master will increase coverage by 0.72%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1172      +/-   ##
==========================================
+ Coverage   54.44%   55.17%   +0.72%     
==========================================
  Files          41       42       +1     
  Lines        9141     9581     +440     
==========================================
+ Hits         4977     5286     +309     
- Misses       3159     3271     +112     
- Partials     1005     1024      +19     

Impacted Files	Coverage Δ
msg_helpers.go	59.29% <0.00%> (-4.72%)	⬇️
ztypes.go	50.46% <0.00%> (-0.50%)	⬇️
zmsg.go	36.11% <0.00%> (-0.05%)	⬇️
types.go	75.74% <0.00%> (ø)
svcb.go	87.66% <0.00%> (ø)
client.go	68.92% <0.00%> (+0.17%)	⬆️
scan.go	84.13% <0.00%> (+0.29%)	⬆️
msg.go	74.56% <0.00%> (+0.76%)	⬆️
zduplicate.go	4.04% <0.00%> (+0.77%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 68df440...f71a0a4. Read the comment docs.