fixing segment io settings bug

* Moving to the auth tokens section
* Removing the feature flag logic since all features are loaded earlier
```
    for feature, value in ENV_TOKENS.get('MITX_FEATURES', {}).items():
        MITX_FEATURES[feature] = value
```
moving settings into the auth tokens section
Merge pull request #180 from edx/jarv/fix-segment-io

fixing segment io settings bug