Accepts addons specified as list.

Some folks specify a list in their addons section, causing a nasty
error message to show up in the build logs. This change accepts
both a hash and a list.

Both of these will work:

addons:
  - firefox: 19

and

addons:
  firefox: 19
Merge pull request #183 from travis-ci/mm-fix-addons-with-list

Accept addons specified as list.