Unify properties for jitpack and version

[david4096]

The important hand edited properties are now editable in one place (parent/pom.xml).

The schema version was being set in multiple places. Added a brief comment that explains how to switch to your own repo for schemas. Place the versioning in a single property so it is reused throughout. Add docs for usage

Pushes version to 0.6.0a7 and adds version as property of parent/pom.xml. @bwalsh

[bwalsh]

+1 thank you for the docs/UpdatingTheSchemas.md document

[david4096]

Depends on https://github.com/ga4gh/compliance/issues/224

[dcolligan]

+1 after squash