Merge pull request #2449 from jsomers/patch-1

Update os-x.md
Upgrade to exception-reporting@0.18.0
Upgrade to welcome@0.15.0
Upgrade to atom-dark-ui@0.29.0
Upgrade to welcome@0.16.0
Merge remote-tracking branch 'origin/master' into cj-add-python-check
:checkered_flag: Verify python version for build

References:
https://github.com/TooTallNate/node-gyp/blob/56dcb889091a6a0594854ccc517e498b21078d5f/lib/configure.js
Merge branch 'jugglingnutcase-exec-python-for-version' into cj-add-python-check
Rename postVerification method to bootstrap
Mimic Atom’s if/else style convention
Reuse pythonExecutable variable
Update error messages