Use more semantic method to authenticate password in SecurePassword

``` diff
-        BCrypt::Password.new(password_digest) == unencrypted_password && self
+        BCrypt::Password.new(password_digest).is_password?(unencrypted_password) && self
```

The original `==` method looks suspicious because it looks like the unencrypted password is being compared to a hashed password. The bcrypt-ruby gem provides an alias `is_password?` that reads better IMHO.

Feel free to close if this was already considered and `==` was chosen. :smile: