Make railites a hard dep

This fixes problems with the `creds` command in msfconsole caused by a load order issue that resulted in `Concern`s not being run on `Mdm` models.

# Pre-verification Steps

- [x] Land rapid7/metasploit_data_models#79 *first*
- [x] Update `metasploit-concern` to released version in `metasploit-credential.gemspec`
- [x] Update `metasploit-model` to released version in `metasploit-credential.gemspec`
- [x] Update `metasploit_data_models` to released version in `metasploit-credential.gemspec`

# Verification

## `metasploit-credential`

### specs
- [x] `rake spec`
- [x] VERIFY no failures

## `metasploit-framework`

- [x] Update `metasploit-concern` to released version in `metasploit-framework.gemspec`
- [x] Update `metasploit-model` to released version in `metasploit-framework.gemspec`
- [x] Update `metasploit_data_models` to released version in `Gemfile`
- [x] Update `metasploit-credential` to new pre-release version that depends on released versions of other gems in `Gemfile`
- [x] `bundle install`
- [x] start msfconsole
- [x] `creds add-password bob P@ssw0rd contosso`
- [x] `creds`
- [x] **VERIFY** output contains the password you just added

# Post-merge Steps

Perform these steps prior to pushing to master or the build will be broke on master.

## Version
- [ ] Edit `lib/metasploit/model/version.rb`
- [ ] Remove `PRERELEASE` and its comment as `PRERELEASE` is not defined on master.

## Gem build
- [ ] gem build *.gemspec
- [ ] VERIFY the gem has no '.pre' version suffix.

## RSpec
- [ ] `rake spec`
- [ ] VERIFY version examples pass without failures

## Commit & Push
- [ ] `git commit -a`
- [ ] `git push origin master`

# Release

## JRuby
- [ ] `rvm use jruby@metasploit-model`
- [ ] `rm Gemfile.lock`
- [ ] `bundle install`
- [ ] `rake release`

## MRI Ruby
- [ ] `rvm use ruby-1.9.3@metasploit-model`
- [ ] `rm Gemfile.lock`
- [ ] `bundle install`
- [ ] `rake release`