Gem skeleton and 3rd-party integrations

MSP-11132

The verification steps mirror those in [Metasploit gems from scratch](https://community.rapid7.com/community/metasploit/blog/2014/09/16/metasploit-gems-from-scratch)

# Verification Steps
- [x] `rvm use ruby-2.1@metasploit-cache`
- [x] `bundle install`

## RSpec
- [x] `rake spec`
- [x] VERIFY no errors
- [x] VERIFY slowest examples were shown
- [x] VERIFY random seed was used

## Simplecov
- [x] `rake spec`
 -[x] VERIFY coverage is printed at end of rspec output
- [x] VERIFY coverage report generated: `open coverage/index.html`

## YARD
- [ ] `rake yard`
- [x] VERIFY no undocumented Modules, Classes, Constants, or Methods.
- [ ] VERIFY `100.0% documented`
- [ ] `open doc/frames.html`
- [ ] VERIFY README renders correctly in YARD docs
- [ ] Goto index
- [ ] Click CHANGELOG
- [ ] VERIFY CHANGELOG renders correctly
- [ ] Goto index
- [ ] Click CONTRIBUTING
- [ ] VERIFY CONTRIBUTING rendered correctly
- [ ] Goto index
- [ ] Click LICENSE
- [ ] VERIFY LICENSE rendered correctly
- [ ] Goto index
- [ ] Click RELEASING
- [ ] VERIFY RELEASING rendered correctly
- [ ] Goto index
- [ ] Click UPGRADING
- [x] VERIFY UPGRADING rendered correctly

## Travis-ci

### Integration
- [x] VERIFY there was a travis-ci build for this PR
- [x] VERIFY build was successful for all environments and rubies

### Badge
- [ ] VERIFY 'build status' badge in README links to builds for this repository

## CodeClimate

### Integration
- [ ] Goto https://codeclimate.com/dashboard
- [ ] VERIFY in the 'Open Source' group that 'rapid7/metasploit-cache' is listed
- [ ] Click 'rapid7/metasploit-cache'
- [ ] Click 'branches'
- [ ] VERIFY this is branch is listed: 'feature/MSP-11132/skeleton'

### Badge
- [x] VERIFY 'codeclimate' badge  in README links to data for this repository

## Coveralls

### Integration
- [x] Goto https://coveralls.io/
- [x] VERIFY rapid7/metasploit-cache is listed
- [ ] VERIFY https://coveralls.io/r/rapid7/metasploit-cache shows build for this branch
- [ ] VERIFY [latest build](https://coveralls.io/builds/1613994) shows coverage reports

### Badge
- [ ] VERIFY 'coverage' badge in README links to coverage reports for this repository

## Gemnasium

### Integration
- [x] Goto https://gemnasium.com/dashboard
- [ ] VERIFY 'METASPLOIT-CACHE' is listed in Trevor Rosen's projects.

### Badge
- [ ] VERIFY 'dependencies' badge links to Gemnasium for this repository.

## Rubygems

### Badge
- [ ] VERIFY gem version badge links to https://rubygems.org/gems/metasploit-cache, but that's it's a 404 because metasploit-cache hasn't been released yet.

## Inch CI

### Integretion
- [ ] VERIFY webhook is setup in repository settings for Inch CI

### Badge
- [ ] VERFY doc badge links to this repository on inch-ci.org

## PullReview

### Integration
- [ ] Goto https://www.pullreview.com/
- [ ] Login with github
- [ ] VERIFY you can see https://www.pullreview.com/github/rapid7/metasploit-cache/reviews/feature/MSP-11132/skeleton
- [ ] VERIFY you can see https://www.pullreview.com/github/rapid7/metasploit-cache/reviews/master

### Badge
- [ ] VERIFY Pull Review badge links to master branch for this repository

## gemspec
- [ ] VERIFY summary is set
- [x] VERIFY hompage is set to this repositoy
- [ ] VERIFY license is BSD-3-clause

## LICENSE
- [ ] VERIFY copyright year is correct
- [ ] VERIFY license is the BSD-3-clause used by Rapid7 for open source

## README.md
- [ ] VERIFY description in README matches description in gemspec