Export fails

[Ethan1983]

Hello,
Thanks for the extension. However, my export is failing in macOS with this error. Are there any other setup instructions?

Traceback (most recent call last):
File "android_export.py", line 185, in
if options.optimize and not checkForPath("optipng"):
File "android_export.py", line 35, in checkForPath
], stdout=DEVNULL, stderr=subprocess.STDOUT)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 522, in call
return Popen(*popenargs, **kwargs).wait()
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 710, in init
errread, errwrite)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1335, in _execute_child
raise child_exception
OSError: [Errno 2] No such file or directory

[Ethan1983]

This patch fixes it for me.

#8