Fix argument handling in main rbenv command

- Explicitly asking for help with `-h` or `--help` exits with 0 status
  and displays help on stdout.

- Not providing any arguments to rbenv results in failure status and
  displays version and help on stderr.

test/rbenv.bats

@@ -4,7 +4,7 @@ load test_helper
 
 @test "blank invocation" {
   run rbenv
-  assert_success
+  assert_failure
   assert_line 0 "$(rbenv---version)"
 }