Support outbound proxy for saucelabs session update PUT req

[Maciek416]

This PR performs two updates:

• Switches from https to request for the saucelabs session update PUT request at the end of a test (to allow for easy proxy support).
• Adds proxy support in the above request if SAUCELABS_OUTBOUND_PROXY is set (see existing README).

/cc @archlichking

[archlichking]

👍

[codecov-io]

Codecov Report

Merging #14 into master will increase coverage by 0.39%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #14      +/-   ##
=========================================
+ Coverage    88.3%   88.7%   +0.39%     
=========================================
  Files           9       9              
  Lines         479     478       -1     
  Branches       89      91       +2     
=========================================
+ Hits          423     424       +1     
+ Misses         56      54       -2

Impacted Files	Coverage Δ
lib/executor.js	72.72% <0%> (+1.72%)	:arrow_up:

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