cost: fix issue #325
BUGFIX: Don't force system ruby

Dbomatic would only use system ruby before, which caused it to fail if
you were using a modern ruby via rvm or rbenv.
Merge pull request #336 from jguiditta/fix_dbomatic_ruby

BUGFIX: Don't force system ruby
Merge pull request #331 from martinpovolny/max_price

cost: fix issue #325