Setup and document local test coverage analysis

[RolandBurrows]

Short description of what this PR does:

Test coverage analysis was only available through CI by default:

# ./test/test_helper.rb

if ENV['CI'] == 'true'
  require 'simplecov'
  SimpleCov.start
end

But now there's a convenient way to access it locally:

ruby-http-client$ rake test:coverage

<lots of debug info>
225 / 239 LOC (94.14%) covered

Note: someone from SendGrid with admin access should add the Test Coverage badge to the main README to show off that sweet 94%.

Checklist

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

[codecov[bot]]

Codecov Report

Merging #99 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #99   +/-   ##
=======================================
  Coverage   95.60%   95.60%           
=======================================
  Files           2        2           
  Lines         341      341           
=======================================
  Hits          326      326           
  Misses         15       15           

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[eshanholtz]

Closing this PR as we now use Codecov for local test coverage.