[gemspec] Remove 1.1.0 post install message

There was a 3 month window between 1.1.0 and 1.1.1 where someone could
rely on the backwards incompatible change introduced in 1.1.0 that was
reverted in 1.1.1. Since it's almost been a year since 1.1.1 was
released this warning should not apply to anyone; we can remove it.
Merge branch 'maint/master/gemspec-1.1.0-install-warning'