Use only one deb line in /etc/apt

This prevents the script from filling up /etc/apt/sources.list with more than one deb line which cause a warning when updating.
strings.Split may return an empty string on no match

* This fixes an index out of range crash if cgroup memory is not
  enabled.
correct documentation for where images are stored on filesystem.
Merge pull request #508 from bdon/master

* docs: doc fix
Check that the line is valid with 6 parts after split
Merge pull request #487 from brunoqc/patch-2

* vagrant: Use only one deb line in /etc/apt
Fix a bug in the Makefile which caused dependency download to fail
Merge pull request #501 from losinggeneration/fix_cgroup_memory_check

- runtime: strings.Split may return an empty string on no match