Enabling email notifications caues mattermost not to start

Thanks for the report @vorgusa There was a bug where an invalid json field in the config file or a failed connection with EmailSettings may cause this error. It's fixed on master and will be in for the next release.

In the meantime, you may try the following steps as a workaround:

1. Stop the server
2. Remove EmailSettings from your config.json
3. Start the server
4. From the System Console re-add the Email Settings