Use stdlib NetworkOptions to locate system cert file

[dhanak]

Fixes #200.

[codecov]

Codecov Report

Merging #262 (bc0c893) into master (e224150) will decrease coverage by 0.07%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
- Coverage   73.56%   73.49%   -0.07%     
==========================================
  Files          12       12              
  Lines         715      732      +17     
==========================================
+ Hits          526      538      +12     
- Misses        189      194       +5     

Impacted Files	Coverage Δ
src/MbedTLS.jl	98.24% <ø> (+0.03%)	⬆️
src/ssl.jl	70.84% <45.45%> (-1.54%)	⬇️
src/ctr_drbg.jl	85.18% <0.00%> (+3.70%)	⬆️
src/entropy.jl	84.61% <0.00%> (+3.84%)	⬆️
src/error.jl	41.17% <0.00%> (+7.84%)	⬆️
src/debug.jl	71.42% <0.00%> (+71.42%)	⬆️

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[quinnj]

Thanks