activate: display setup instructions only with PYENV_VIRTUALENV_INIT=0

This fixes displaying the note, in case "init" has been run manually
and/or the init is not in the expected place.

It removes the unnecessary calls to "grep".

pyenv-virtualenv-init will look at `$PYENV_SHELL` now, too.

Tests have been adjusted/fixed.

test/init.bats

@@ -3,6 +3,7 @@
 load test_helper
 
 @test "detect parent shell" {
+  unset PYENV_SHELL
   root="$(cd $BATS_TEST_DIRNAME/.. && pwd)"
   SHELL=/bin/false run pyenv-virtualenv-init -
   assert_success