Add support for Go 1.15 to CI

[asahasrabuddhe]

Since Go 1.15 is released, we should add support for testing against that version. Also, since Go 1.12 is not longer supported, it is removed from the matrix.

[codecov]

Codecov Report

Merging #1683 (fdbcc2f) into master (7a90304) will increase coverage by 0.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1683      +/-   ##
==========================================
+ Coverage   84.35%   84.72%   +0.37%     
==========================================
  Files          28       29       +1     
  Lines        1911     1938      +27     
==========================================
+ Hits         1612     1642      +30     
+ Misses        189      188       -1     
+ Partials      110      108       -2     

Impacted Files	Coverage Δ
middleware/decompress.go	94.73% <0.00%> (ø)
middleware/cors.go	85.52% <0.00%> (+7.58%)	⬆️

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[lammel]

Thanks @asahasrabuddhe for adding go1.15.
I'd like to keep go 1.12 just for a few months longer as not all developers have moved yet probably.

Please adjust, I'll merge then.

[asahasrabuddhe]

Thanks @asahasrabuddhe for adding go1.15.
I'd like to keep go 1.12 just for a few months longer as not all developers have moved yet probably.

Please adjust, I'll merge then.

Done