Add standard git ignores

MSP-9815
Fix capitalization

MSP-9815

bundle gem turned metasploit-erd into Metasploit::Erd, instead of the
desired Metasploit::ERD, so fix capitalization on Erd by making it ERD.
Semantic Versioning 2.0.0

MSP-9815
simplecov

MSP-9815
coveralls

MSP-9815
travis-ci badge

MSP-9815
CodeClimate badge

MSP-9815
Coveralls.io badge

MSP-9815
Gemnasium badge

MSP-9815
Rubygems badge

MSP-9815
Add semantic version restrictions to gemspec

MSP-9815

Add `~>` versions to eliminate warnings from `gem build`.
Add correct BSD 3-clause license

MSP-9815

Metasploit projects use the BSD 3-clause instead of the default MIT
license.
YARD

MSP-9815
Push activesupport in specs

MSP-9815

`#blank?` is only used in version_spec.rb, so activesupport only needs
to be required in test and required explicitly in version_spec.rb.
Add description to README from gemspec

MSP-9815
Merge pull request #1 from rapid7/feature/gem-skeleton

Gem skeleton
Merging and saving questions of test/doc deps for later