Change logout methods to return a promise

[ryanclark]

Checklist

• Issue number for this PR: Suggestion: logout to return a promise #583 (required)
• Docs included?: yes
• Test units included?: no
• e2e tests included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

• Change the AuthBackend.unauth() method to return the value of the _authBackend.unauth call
• Change the FirebaseSdkAuthBackend.unauth() method to return a promise instead of void to match the API as suggested in Suggestion: logout to return a promise #583

Code sample

this.auth$.logout().then(() => {
  console.log('Logged out!');
});

[ryanclark]

Yarn doesn't seem to be playing nicely with Travis which has led the build to fail - https://travis-ci.org/angular/angularfire2/builds/176247589

[katowulf]

@davideast Travis error looks related to #611; ideas?

[ryanclark]

@davideast is it possible to get this reviewed soon please? :)

[davideast]

@rynclark Thanks for getting this in!

[lednhatkhanh]

Thanks, waiting for this to be merged!
Oh, I saw that it has been merged, how can I update angularfire 2 to have this function?