fix 1.6 support

[Roger-luo]

fix #183

[codecov]

Codecov Report

Merging #185 (12e9031) into master (e94871b) will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
+ Coverage   82.57%   82.87%   +0.30%     
==========================================
  Files          19       19              
  Lines        1280     1285       +5     
==========================================
+ Hits         1057     1065       +8     
+ Misses        223      220       -3     

Impacted Files	Coverage Δ
src/codegen/julia.jl	86.99% <100.00%> (ø)
src/compat.jl	100.00% <100.00%> (+100.00%)	⬆️
src/frontend/markdown.jl	94.11% <100.00%> (ø)
src/builder/sysimg.jl	84.61% <0.00%> (+1.53%)	⬆️

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