Coveralls: fix a deprecated HTTP post call

[DilumAluthge]

Fixes #308

[coveralls]

Pull Request Test Coverage Report for Build 934165440

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.5%) to 90.686%

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Files with Coverage Reduction	New Missed Lines	%
src/codecovio.jl	2	86.87%
src/coveralls.jl	2	94.29%

Totals
Change from base Build 934154514:	0.5%
Covered Lines:	185
Relevant Lines:	204

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💛 - Coveralls

[codecov]

Codecov Report

Merging #321 (b3394b9) into master (ce273bc) will increase coverage by 0.53%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
+ Coverage   90.14%   90.68%   +0.53%     
==========================================
  Files           2        2              
  Lines         203      204       +1     
==========================================
+ Hits          183      185       +2     
+ Misses         20       19       -1     

Impacted Files	Coverage Δ
src/coveralls.jl	94.28% <100.00%> (-1.87%)	⬇️
src/codecovio.jl	86.86% <0.00%> (+3.03%)	⬆️

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