feat: upgrade to go 1.15

[lucaskanashiro]

This issue was found in Ubuntu as you can see here:

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-hirsute/hirsute/amd64/g/golang-github-caarlos0-env/20210104_100201_2ced3@/log.gz

[caarlos0]

its because we're still building with go 1.14, and this changed in go 1.15... can you also change here to 1.15?

Then the tests should pass I think

[codecov]

Codecov Report

Merging #150 (d3c4e21) into master (80e991b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   98.31%   98.31%           
=======================================
  Files           1        1           
  Lines         237      237           
=======================================
  Hits          233      233           
  Misses          2        2           
  Partials        2        2           

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[caarlos0]

Thanks!