Normalize eventTime format to iso8601

[shouya]

According to the PredictionIO EventServer documentation. The time must be in ISO8601 format, otherwise the event will not be successfully created.

Before:

[9] pry(main)> x = PIO::EventClient.set_user(111)
  HTTP POST (29.63ms)   http://192.168.20.4:7070/events.json?accessKey=<masked>
  Request body   {"eventTime":"2017-07-24 14:43:26","event":"$set","entityType":"user","entityId":111}
  Response status   Net::HTTPBadRequest (400)
  Response body   {"message":"org.json4s.package$MappingException: Fail to extract eventTime 2017-07-24 14:43:26"}
PredictionIO::EventClient::NotCreatedError: {"message":"org.json4s.package$MappingException: Fail to extract eventTime 2017-07-24 14:43:26"}
from /Users/shou/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/predictionio-0.9.6/lib/predictionio/event_client.rb:333:in `sync_events'

Where it should be:

[10] pry(main)> x = PIO::EventClient.set_user(111, 'eventTime' => Time.now.iso8601)
  HTTP POST (90.27ms)   http://192.168.20.4:7070/events.json?accessKey=<masked>
  Request body   {"eventTime":"2017-07-24T14:43:48+08:00","event":"$set","entityType":"user","entityId":111}
  Response status   Net::HTTPCreated (201)
  Response body   {"eventId":"Kmgu6x9SvJQtjsepaRECjAAAAV1zVmSghsZcqOIsot8"}
=> #<Net::HTTPCreated 201 Created readbody=true>

[dszeto]

Thanks for the fix @shouya ! Sorry that it took a bit to merge.