Failed to clone repo

Hi,

I'm getting the following error in Travis and local as well:

```
Cloning into '/tmp/pr-bot/ggPjZc'...
remote: Invalid username or password.
fatal: Authentication failed for 'http://myRepo....'
```

It's a private repo I'm trying to run the bot against. I'm wondering if I miss something or it's just some missing config.

My config is basically the one in the docs:

```js
const prbot = require('pr-bot');

module.exports = {
botUsername: 'bot-team',
repoDetails: {
owner: 'mycompany',
repo: 'frontend-service'
},
plugins: [
new prbot.plugins.Size({
globPattern: ['src/**/*.js', 'resources/sn.scss', 'resources/sn-simple.scss'],
globOptions: {
ignore: [
'**/node_modules/**/*'
]
}
})
]
};
```

Thanks! :)