Support composer/ca-bundle fallback

[colinodell]

Fixes #90

This is one of three possible solutions to #90 (see #103 and #104). This one can be included with either #103 or #104 if you'd like.

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• If composer/ca-bundle is installed, it'll use that to locate a CA bundle and configure curl to use that. This will be either the system's default, or Mozilla's if none is found

[codecov]

Codecov Report

Merging #105 into master will decrease coverage by 2.31%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage    84.7%   82.38%   -2.32%     
==========================================
  Files           3        3              
  Lines         170      176       +6     
==========================================
+ Hits          144      145       +1     
- Misses         26       31       +5

Impacted Files	Coverage Δ
lib/Client.php	78.91% <16.66%> (-2.65%)	⬇️

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[colinodell]

This change has been merged (via cherry-pick) into #103