OBLIGATORY_CREDS_KEYS.keys is not a function
OBLIGATORY_CREDS_KEYS.keys is not a function

Please check this Issue
https://github.com/expo/expo/issues/2014
```bash
 henry@hmp  ~/work/st/dev  node
> const OBLIGATORY_CREDS_KEYS = {
...   pushP12: 'pushCert',
...   pushPassword: 'pushCert',
...   provisioningProfile: 'provisioningProfile',
...   teamId: 'teamId',
... };
undefined
> OBLIGATORY_CREDS_KEYS.keys()
TypeError: OBLIGATORY_CREDS_KEYS.keys is not a function
> Object.keys(OBLIGATORY_CREDS_KEYS)
[ 'pushP12', 'pushPassword', 'provisioningProfile', 'teamId' ]
>
```

Closes expo/exp#107

fbshipit-source-id: 5872243

exponentjs/exp