Upgrade build to Go 1.15

[yurishkuro]

Upgrade build to Go 1.15

[codecov]

Codecov Report

Merging #539 into master will decrease coverage by 0.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
- Coverage   89.23%   88.25%   -0.99%     
==========================================
  Files          61       61              
  Lines        3919     3277     -642     
==========================================
- Hits         3497     2892     -605     
+ Misses        295      259      -36     
+ Partials      127      126       -1     

Impacted Files	Coverage Δ
internal/baggage/remote/options.go	71.42% <0.00%> (-7.15%)	⬇️
transport/zipkin/http.go	70.37% <0.00%> (-6.11%)	⬇️
testutils/udp_transport.go	52.38% <0.00%> (-5.69%)	⬇️
transport/http.go	79.62% <0.00%> (-4.20%)	⬇️
utils/localip.go	66.66% <0.00%> (-3.93%)	⬇️
utils/http_json.go	66.66% <0.00%> (-3.93%)	⬇️
log/zap/field.go	84.21% <0.00%> (-3.16%)	⬇️
crossdock/client/client.go	72.72% <0.00%> (-2.89%)	⬇️
crossdock/client/trace.go	66.66% <0.00%> (-2.65%)	⬇️
interop.go	66.66% <0.00%> (-2.57%)	⬇️
... and 51 more

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