log options timestamp is ignored

[EdwardDiehl]

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]