Avoid invoking hash -r in fish

It doesn't exist as a builtin, and it doesn't seem there is a way to
detect support for a shell builtin that is portable. So, just detect
fish and don't the rehash command at all.

Fixes #478

test/rehash.bats

@@ -109,6 +109,6 @@ SH
 @test "sh-rehash in fish" {
   create_executable "2.0" "ruby"
   RBENV_SHELL=fish run rbenv-sh-rehash
-  assert_success "hash -r 2>/dev/null ; or true"
+  assert_success ""
   assert [ -x "${RBENV_ROOT}/shims/ruby" ]
 }