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

libexec/rbenv-sh-rehash

@@ -15,11 +15,9 @@ rbenv-rehash
 
 case "$shell" in
 fish )
-  or='; or'
+  # no rehash support
   ;;
 * )
-  or='||'
+  echo "hash -r 2>/dev/null || true"
   ;;
 esac
-
-echo "hash -r 2>/dev/null $or true"