Move Chrome's dmg_package source, checksum & dmg_name to attributes file

    Notes:

      To use a custom version, put something like this in your
      soloistrc:

        node_attributes:
          sprout:
            chrome:
              dmg:
                source: https://dl.google.com/chrome/mac/stable/GGRO/googlechrome.dmg
                checksum: 6f4673c02ec38d4d8a480dc1e41caa97f49320ed9898bcbe1abde3272b636061
                dmg_name: googlechrome
Updating default chrome installed to 28.0.1500.71
Merge remote-tracking branch 'origin/pr/83'