Add simplecov to cucumber env

MSP-11647

Port simplecov setup from metasploit-version so that child processes, in
this case msfconsole, will load simplecov and report coverage back to
the parent process.  Coverage from msfconsole's cucumber features is the
only way to get coverage on portions of the command dispatcher commands.

# Verification Steps
- [ ] `rm -rf coverage`
- [ ] `rake cucumber`
- [ ] VERIFY no failures
- [ ] VERIFY scenario names are listed in coverage message *order may differ*: `Coverage report generated for Cucumber Features, Help command The 'help' command's output`
- [ ] `open coverage/index.html`
- [ ] Type `dispatcher_shell` in the search box 
![screen shot 2014-11-24 at 8 28 37 am](https://cloud.githubusercontent.com/assets/2230482/5166235/f1715526-73b3-11e4-9b73-9de919be12de.png)
- [ ] Click `lib/rex/ui/text/dispatcher_shell.rb`
![screen shot 2014-11-24 at 8 34 23 am](https://cloud.githubusercontent.com/assets/2230482/5166337/be46383c-73b4-11e4-99f7-192005e0550e.png)
- [ ] Use Cmd+F/Ctrl+F to find `cmd_help`
- [ ] VERIFY line 174 is covered
![screen shot 2014-11-24 at 8 35 24 am](https://cloud.githubusercontent.com/assets/2230482/5166381/ff6f4a2e-73b4-11e4-9e6a-cfe233bbe60f.png)
- [ ] VERIFY :disappointed:  that majority of `cmd_help` isn't covered