Don't include spec logs in gem

This excludes large (sometimes 300MB+) log files when packaging the metasploit_credential gem.  This should make the gem smaller, quicker to unpack and use less RAM when installing.

## Verification Steps
- [ ] Package using the normal process (e.g. `gem build *.gemspec`)
- [ ] Unpack to a temporary directory:  `cd /tmp && gem unpack metasploit-credential*.gem`
- [ ] VERIFY: `spec/dummy/log/test.log` does not exist