Add justify-content: space-evenly

Summary:
Adds new ```space-evenly``` for ```justify-content```.

Also adds a typofix in one of the other justify-content tests.

Fixes #657
Closes https://github.com/facebook/yoga/pull/658

Differential Revision: D6407996

Pulled By: emilsjolander

fbshipit-source-id: cc837409e1345624b4bd72c31e25fe68dcb0f6a3

enums.py

@@ -35,6 +35,7 @@ ENUMS = {
         'FlexEnd',
         'SpaceBetween',
         'SpaceAround',
+        'SpaceEvenly',
     ],
     'Overflow': [
         'Visible',