Failure to define :sentinels option results in error when calling 'slaves'

[phallstrom]

I realize this is an edge case, but in development I have no sentinels so left off the :sentinels option entirely. For kicks I then called slaves on my redis object and got this error:

NoMethodError: undefined method `shift' for nil:NilClass
from .../gems/redis-sentinel-1.5.0/lib/redis-sentinel/client.rb:56:in `try_next_sentinel'

From reading the source I think tweaking parse_sentinel_options to return an empty array if nothing is provided would fix it.

See: https://github.com/flyerhzm/redis-sentinel/blob/master/lib/redis-sentinel/client.rb#L201

I'm happy to make the change and add specs if you agree this is the right behavior.