Raise version number to 7.6.4.

Resolves: #78369
Releases: master,7.6
Change-Id: Iabaddaa0c0223f7a1564e1ea0ec2d4c6d2e523e9
Reviewed-on: https://review.typo3.org/50305


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>