Track upstream versions, now 3.1.6
Update version.rb

This change takes into account @ketan21 pull request, so
gem version is bumped to 0.0.8 instead of 0.0.7.
Update d3 version in README to v3.1.6
Merge pull request #14 from CodeCantor/v3.1.6

Update d3 version to 3.1.6