Update Go version to 1.15

[drgarcia1986]

[codecov-io]

Codecov Report

Merging #646 into master will decrease coverage by 2.51%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
- Coverage   50.31%   47.79%   -2.52%     
==========================================
  Files          86       86              
  Lines        6245     5469     -776     
==========================================
- Hits         3142     2614     -528     
+ Misses       2889     2645     -244     
+ Partials      214      210       -4     

Impacted Files	Coverage Δ
pkg/client/cmd/version.go	25.00% <0.00%> (-15.00%)	:arrow_down:
pkg/client/cmd/completion.go	25.00% <0.00%> (-15.00%)	:arrow_down:
pkg/server/uid/gen.go	50.00% <0.00%> (-10.00%)	:arrow_down:
pkg/server/deploy/responses.go	66.66% <0.00%> (-7.25%)	:arrow_down:
pkg/server/secrets/file.go	55.55% <0.00%> (-5.74%)	:arrow_down:
pkg/server/team/team.go	63.04% <0.00%> (-5.48%)	:arrow_down:
pkg/client/client.go	5.55% <0.00%> (-4.45%)	:arrow_down:
pkg/server/teresa_errors/errors.go	85.71% <0.00%> (-4.29%)	:arrow_down:
pkg/server/exec/handlers.go	45.83% <0.00%> (-4.17%)	:arrow_down:
pkg/server/app/app.go	66.01% <0.00%> (-3.67%)	:arrow_down:
... and 76 more

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