[CHEF-4371] Issue with freebsd package provider when package_name contains a plus

http://tickets.opscode.com/browse/CHEF-4371

The regular expression being used to determine the installed version of a 
package would get tripped up when the package name included plus signs
  Example: bonnie++

I suspect that other regular expression characters in the package_name 
would also cause problems.

This solution avoids regular expressions by using slice and split to extract the
portion of the string that is required.