Update gemspec to require mail 2.4.4 that fixes security issues

There are some security issues in the mail gem that were fixed in 2.4.4 (directory traversal and execution of arbitrary shell commands when using sendmail/exim).

This was fixed in rails master (24d244c1bc1a744de599df18d74c9b343fd4c9fe), 3-1-stable (d3dc2a7a7d5c54547c481d726ca61d7a0e06c3c4), and in 3-2-stable (74b782999f7f2c8913e79e2c65366429b9a3e65e).

I suggest to include this in 3-0-stable as well as the mail gem can't be updated due to the "restrictive" version constraint in actionmailer.

For reference:
http://www.osvdb.org/show/osvdb/81631 (CVE 2012-2139)
http://www.osvdb.org/show/osvdb/81632 (CVE 2012-2140)


Best regards
Joerg