RELEASE-NOTES: Don't imply that HHVM 3.1 is supported

RELEASE-NOTES: Don't imply that HHVM 3.1 is supported

HHVM 3.1 is no longer supported by Facebook, not packaged by any Linux
distribution I know of, and MediaWiki is not routinely tested with it
(e.g. neither Wikimedia Jenkins nor Travis CI runs automated tests
using HHVM 3.1).

The Compatibility section already has a note that "There is experimental
support for HHVM 3.6.5 or later." So instead just refer to that section.
(HHVM 3.6 is also no longer supported by Facebook or still used by
Wikimedia, as far as I know, though that's a separate matter.)

Change-Id: I62352f0edc6b9f95acaaef4cb717990e52db1146