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

javascript/sources/YGEnums.js

@@ -52,12 +52,13 @@ module.exports = {
   FLEX_DIRECTION_ROW: 2,
   FLEX_DIRECTION_ROW_REVERSE: 3,
 
-  JUSTIFY_COUNT: 5,
+  JUSTIFY_COUNT: 6,
   JUSTIFY_FLEX_START: 0,
   JUSTIFY_CENTER: 1,
   JUSTIFY_FLEX_END: 2,
   JUSTIFY_SPACE_BETWEEN: 3,
   JUSTIFY_SPACE_AROUND: 4,
+  JUSTIFY_SPACE_EVENLY: 5,
 
   LOG_LEVEL_COUNT: 6,
   LOG_LEVEL_ERROR: 0,