Enable ci.sh to run on osx

[springmeyer]

This prepares ci.sh to run on OS X. Per chat with @lucaswoj we are going to try using OS X machines on circleci. If this works this could provide a fallback if headless-gl stability on linux continues to be a problem. This can be tested by:

• merging this PR
• going to https://circleci.com/gh/mapbox/mapbox-gl-js/edit#build-environment and flipping the build environment to OS X

@lucaswoj - let's sync up tomorrow morning before enabling.

[lucaswoj]

This looks great! I'll put something on your calendar for tomorrow morning.

Is the idea that we'll remove the if [[ $(uname -s) == 'Linux' ]]; then blocks once we switch CircleCI over to OSX? Or should we keep those for debugging?

[mourner]

Let's try it out!

[mourner]

Hmm, it failed starting with the following error in the "configure build" section:

No shared schemes were found in your repo. Please ensure you have shared your
  scheme. See the iOS documention for more information on sharing schemes.
  Alternatively, you can explicitly set a scheme with the 'XCODE_SCHEME'
  environment variable.

Action failed: Configure the build

Will switch it back for the time being, let's investigate.

[springmeyer]

Hmm, it failed starting with the following error in the "configure build" section:

Fix for that now over at #2965