Speed up scaling with gm

[zinserjan]

Previously scaling of screenshots was always done via Jimp. This PR enables scaling via GraphicsMagick when available.

Stats:

• url: https://github.com/zinserjan/wdio-screenshot
• method: saveDocumentScreenshot
• Measured before and after makeAreaScreenshot with console.time

	Before #69	After #69	Now	Improvement (total)
iPad Air 2 iOS 10.2 landscape	10825ms	6888ms	5881ms	45%
Chrome v56 1024x768	6328ms	5184ms	2180ms	65%
Firefox v51 1024x768	1890ms	1485ms	1477ms	no difference*

• Firefox already emits non-retina images, that's why this PR does not have any impacts on the performance