Make the gemspec compatible with JRuby and enable spec runs under JRuby

1. Removed sqlite3 from the gemspec
2. Added sqlite3 and activerecord-jdbcsqlite3-adapter with appropriate platform directives to each of the gemfiles.

Specs now run under JRuby, although they fail for the reasons discussed in issue #393

Specs don't run under 1.8.7 and ree, which is the case in current master - https://travis-ci.org/petergoldstein/authlogic/builds/19884461