Update docs branch ref from 6.x to 6.7
:version: 6.7.0
-:major-version: 6.x
+:major-version: 6.7
:lucene_version: 7.7.0
:lucene_version_path: 7_7_0
-:branch: 6.x
+:branch: 6.7
:jdk: 1.8.0_131
:jdk_major: 8
:build_flavor: default
