log options timestamp is ignored

The `timestamp` passed to the `log` method via options here
https://github.com/logdna/ruby/blob/v1.5.0/lib/logdna.rb#L79-L81

is ignored here
https://github.com/logdna/ruby/blob/v1.5.0/lib/logdna/client.rb#L64

Seems that this line should be changed to something like `timestamp: opts[:timestamp]`