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.

This should shrink the gem size from about 15MB to about 100K.

## Verification Steps
- [x] Package using the normal process (e.g. `gem build *.gemspec`)
- [x] Unpack to a temporary directory:  `cd /tmp && gem unpack metasploit-credential*.gem`
- [x] VERIFY: `spec/dummy/log/test.log` does not exist

# Post-merge Steps

Perform these steps prior to pushing to master or the build will be broke on master.

## Version
- [x] Edit `lib/metasploit/credential/version.rb`
- [x] Remove `PRERELEASE` and its comment as `PRERELEASE` is not defined on master.

## Gem build
- [x] gem build *.gemspec
- [x] VERIFY the gem has no '.pre' version suffix.

## RSpec
- [x] `rake spec`
- [x] VERIFY version examples pass without failures

## Commit & Push
- [x] `git commit -a`
- [ ] `git push origin master`