Switch from node-gm to sharp

[JeremyPlease]

In an attempt to speed up some cloud code functions that rely on parse-image, I switched node-gm to sharp.

Some initial, very unscientific benchmarking (ie. resizing 200 images in a loop), show a ~35% increase in speed.

[codecov-io]

Current coverage is 96.66% (diff: 95.08%)

Merging #14 into master will decrease coverage by 0.95%

@@             master        #14   diff @@
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  Files             1          1          
  Lines            84         90     +6   
  Methods          18         19     +1   
  Messages          0          0          
  Branches         22         27     +5   
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+ Hits             82         87     +5   
- Misses            2          3     +1   
  Partials          0          0          

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[funkenstrahlen]

Did you have a look on memory usage? I just switched away from sharp to this repo because sharp did use too much memory.

[JeremyPlease]

@funkenstrahlen I never did any actual benchmarking or look at memory usage. I'm using my sharp branch in production without any issues.

[funkenstrahlen]

I had quite big memory issues with sharp as soon I had to convert +50 images in a row. If your code does not have this problem, I like it. Just test for it please.