Update Ruby version to 2.3.3 in rbenv
Updating .ruby-version which is used by rbenv
On our test systems, we do not use rbenv, so we can still test with: 2.3.1 on test 2.3.3 on test_stretch
Updating .ruby-version which is used by rbenv
On our test systems, we do not use rbenv, so we can still test with: 2.3.1 on test 2.3.3 on test_stretch