[TASK]][!!!] Drop support for TYPO3 6.1

As 6.1 reached EOL, it has already been removed from travis CI.
As a consequence, new updates shouldn't get delivered to 6.1 users.

They can, however, update manually and see if it works.
Merge pull request #690 from cedricziel/drop61fromSupport

[TASK]][!!!] Drop support for TYPO3 6.1