Update version in docs

[purrgrammer]

Currently shows 0.6.3 while the last release is 0.7.3

[fedefernandez]

I've manually updated the documentation with the command:
docs/publishMicrosite

It needs to set a personal token with repo permissions in the ORG_GITHUB_TOKEN env variable as shown in the sbt-microsites docs:

• https://47deg.github.io/sbt-microsites/docs/publish-with-travis.html#publish-using-github4s

Alternatively, you could set the micrositePushSiteWith setting to GHPagesPlugin and it will use your local credentials, as shown in:

• https://47deg.github.io/sbt-microsites/docs/settings.html