Make FacebookUtils work with React Native

[frantic]

When calling Parse.FacebookUtils.init(); from React Native I get this error (Parse v1.6.9)

This pull request fixes it. Not sure how to test if this breaks web behavior in any way.

[codecov-io]

Current coverage is 85.24%

Merging #92 into master will decrease coverage by -0.14% as of 7f9593e

@@            master     #92   diff @@
======================================
  Files           37      37       
  Stmts         2806    2806       
  Branches         0       0       
  Methods          0       0       
======================================
- Hit           2396    2392     -4
  Partial          0       0       
- Missed         410     414     +4

Review entire Coverage Diff as of 7f9593e

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[frantic]

@andrewimm mind taking a look?

[andrewimm]

lgtm ✔️