Update version.rb and version_spec.rb for semantic versioning 2.0

MSP-10011
Update README to mention versioning

MSP-10011
Merge branch 'staging/electro-release' into feature/MSP-10011/semantic-versioning-2-0

MSP-10011
Merge pull request #15 from rapid7/feature/MSP-10011/semantic-versioning-2-0

Semantic versioning 2 0

MSP-10011 #land