Only patch File on JRuby < 1.7.14

MSP-11372

JRuby 1.7.14 fixed the File.realpath bug, so Metasploit::Model::File
only really needs to exist for JRuby < 1.7.14, but since pushing that
logic into other code complicates the code, leave
Metasploit::Model::File, but just make it an alias of ::File when the
patch for JRuby < 1.7.14 is unnecessary.

# Verification

## Specs

### MRI 1.9.3
- [x] `rvm use ruby-1.9.3@metasploit-model`
- [x] `rm Gemfile.lock`
- [x] `bundle install`
- [x] `rake spec`
- [x] VERIFY no failures
- [x] VERIFY `Metasploit::Model::File aliases ::File` test ran
- [x] VERIFY `Metasploit::Model::File#realpath should be necessary because File.realpath does not resolve symlinks` test did NOT run

### JRuby < 1.7.14
- [x] `rvm use jruby-1.7.3@metasploit-model`
- [x] `rm Gemfile.lock`
- [x] `bundle install`
- [x] `rake spec`
- [x] VERIFY no failures
- [x] VERIFY `Metasploit::Model::File aliases ::File` test did NOT run
- [x] VERIFY `Metasploit::Model::File#realpath should be necessary because File.realpath does not resolve symlinks` test ran

### JRuby >= 1.7.14
- [x] `rvm use jruby-1.7.14@metasploit-model` (jruby-1.7.15 is fine too)
- [x] `rm Gemfile.lock`
- [x] `bundle install`
- [x] `rake spec`
- [x] VERIFY no failures
- [x] VERIFY `Metasploit::Model::File aliases ::File` test ran
- [x] VERIFY `Metasploit::Model::File#realpath should be necessary because File.realpath does not resolve symlinks` test did NOT run

# Post-merge Steps

Perform these steps prior to pushing to master or the build will be broke on master.

## Version
- [x] Edit `lib/metasploit/model/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`
- [x] `git push origin master`

# Release

Complete these steps on DESTINATION

## Version

### Compatible changes

- [ ] Increment `PATCH`](lib/metasploit/model/version.rb)

## 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`