Issue title: installation issue on Raspbian
 Issue body: when installing Iris on Raspbian I get the following error:

sudo echo "mopidy ALL=NOPASSWD: /usr/local/lib/python2.7/dist-packages/mopidy_iris/system.sh" >> /etc/sudoers
bash: /etc/sudoers: Permission denied

must be something related to permissions but cannot figure out what; sudo generally works
 Comments: 
 Comment 0: I had the same issue.

Instead you can do:
`sudo visudo`

and then paste the line:
`mopidy ALL=NOPASSWD: /usr/local/lib/python2.7/dist-packages/mopidy_iris/system.sh`
 Comment 1: @mathieuhays thanks, your method worked great
{'source': 'GithubIssues', 'repo_name_with_owner': 'jaedb/Iris', 'issue_id': 328393769, 'repo_stars': 1039.0}