Only depend on rb-readline on :ruby platform

MSP-11585

# Verification Steps

## Linux/OSX

On Linux or OS X.

### rb-readline installed
- [ ] `bundle install`
- [ ] VERIFY `rb-readline` is in `bundle list`

### msfconsole boots
- [ ] VERIFY `msfconsole` boots.

## Windows

On Windows.

### rb-readline NOT installed
- [ ] `bundle install`
- [ ] VERIFY `rb-readline` is NOT in `bundle list`

### msfconsole boots
- [ ] VERIFY `msfconsole` boots.