I hope it's not too late. In any case, for future reference...
Yes, and no. Unless I'm mistaken, there's nothing stopping a white person from becoming Rastafarian. However, this does, in fact, mean a non-believer. From Wikipedia,
For the Rastas the razor, the scissors and the comb are the three Babylonian or Roman inventions. So close is the association between dreadlocks and Rastafari, that the two are sometimes used synonymously. In reggae music, a follower of Rastafari may be referred to simply as a "hairlocks", "dreadlocks" or "natty (natural) dread", whilst those non-believers who cut their hair are referred to as baldheads.
 No, I was wrong about the white part. Upon further reading I have found that the Rastafari movement is Afrocentric. I would imagine now that it would very unlikely for a white person to be accepted by other Rastas.