ci: drop Go-1.9+1.10, add Go-1.12.x and Go-1.13.x

[sbinet]

No description provided.

[ncw]

Great idea - looks good to me :-)

[codecov-io]

Codecov Report

Merging #113 into master will increase coverage by 2.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   69.92%   72.54%   +2.61%     
==========================================
  Files          60       60              
  Lines       10819    11887    +1068     
==========================================
+ Hits         7565     8623    +1058     
- Misses       2729     2739      +10     
  Partials      525      525

Impacted Files	Coverage Δ
parser/lexer.go	89.98% <0%> (+0.01%)	⬆️
parser/y.go	94.74% <0%> (+27.09%)	⬆️

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