Bump docs version to 6.0

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@308462 91177308-0d34-0410-b5e6-96231b3b80d8

docs/conf.py

@@ -47,9 +47,9 @@ copyright = u'2011-2017, LLVM Project'
 # built documents.
 #
 # The short X.Y version.
-version = '5.0'
+version = '6.0'
 # The full version, including alpha/beta/rc tags.
-release = '5.0'
+release = '6.0'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.