Bump ember fetch version, fixes #255

[joshuataylor]

The ember-fetch dependency just needs to be bumped to also allow v6.

Testing:
In your package.json, change ember-simple-auth-token to be:

    "ember-simple-auth-token": "joshuataylor/ember-simple-auth-token#bump-ember-fetch",

Now rerun yarn, and you should be good to test.

[Uysim]

When this gonna merge?

[jpadilla]

@Uysim does this fix your issue on #257 as well?

[Uysim]

I tested it. It didn't fixed my problem

[fenichelar]

Need to update package-lock.json as well.