Fix OSX build

[andig]

fix #4515 as per discussion. I had to add two tweaks to make SublimeText and gulp behave, too.

[simonbrunel]

I think that works, but to be sure, can you push a change that makes a test to fail. I'm curious to see how the Travis build behaves and how errors are reported.

[etimberg]

Looks good. I agree with @simonbrunel's comment about confirming that the CI can still fail

[andig]

Here's the output of a failed test:

Chrome 60.0.3112 (Mac OS X 10.12.6): Executed 574 of 578 (1 FAILED) (skipped 4) ERROR (28.165 secs / 26.772 secs)
Firefox 54.0.0 (Mac OS X 10.12.0): Executed 574 of 578 (1 FAILED) (skipped 4) (34.988 secs / 31.933 secs)
[10:16:27] 'unittest' errored after 45 s
[10:16:27] Error: Karma returned with the error code: 1
    at /Users/andig/Documents/htdocs/Chart.js/gulpfile.js:194:21
    at removeAllListeners (/Users/andig/Documents/htdocs/Chart.js/node_modules/karma/lib/server.js:380:7)
    at Server.<anonymous> (/Users/andig/Documents/htdocs/Chart.js/node_modules/karma/lib/server.js:391:9)
    at Server.g (events.js:260:16)
    at emitNone (events.js:72:20)
    at Server.emit (events.js:166:7)
    at emitCloseNT (net.js:1521:8)
    at nextTickCallbackWith1Arg (node.js:431:9)
    at process._tickCallback (node.js:353:17)

~/Documents/htdocs/Chart.js retina* 50s

Previously, this would show a gulp formatError. The message has changed, gulp will still non-zero error code.