use InsecureSkipVerify for validation

[joejulian]

Description

The server will not yet have a valid certificate so we need to disable
certificate validation in the HTTPGetter.

Fixes #554

[CLAassistant]

All committers have signed the CLA.

[maraino]

@dopey @joejulian See my comment in #554 (comment)

[codecov-commenter]

Codecov Report

Merging #557 (6369a33) into master (582d6b1) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head 6369a33 differs from pull request most recent head 0369151. Consider uploading reports for the commit 0369151 to get more accurate results

@@            Coverage Diff             @@
##           master     #557      +/-   ##
==========================================
- Coverage   76.96%   76.92%   -0.04%     
==========================================
  Files         109      109              
  Lines        9773     9778       +5     
==========================================
  Hits         7522     7522              
- Misses       1868     1873       +5     
  Partials      383      383              

Impacted Files	Coverage Δ
acme/api/handler.go	67.71% <0.00%> (-2.78%)	⬇️

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