Source regardless of g:rainbow_active existence

I think it's kind of unexpected to source the plugin only when `g:rainbow_active` is defined.

If you want to use it sporadically through the `:RainbowToggle` command, having to set `g:rainbow_active` to 0 doesn't really follow the principle of least astonishment.
do you think your parentheses matches?  did you runned your code for test?