Changes Travis, Circle CI, and Semaphore payloads

Payloads now use "started_at" values for published_at when creating project statuses.

There are also some payloads that have no information in their webhook request body to indicate a published_at timestamp. These payloads use Time.now, but could probably be a bit smarter about how they determine their published_at time.

Payloads without designated published_at timestamps:
* https://github.com/pivotal/projectmonitor/blob/master/lib/payload/jenkins_json_payload.rb#L40
* https://github.com/pivotal/projectmonitor/blob/master/lib/payload/team_city_json_payload.rb#L35

Payloads with ambiguous published_at timestamps:
* https://github.com/pivotal/projectmonitor/blob/5d8a5bbf3b13e8d4e68d61ff1cde12c92e933e40/lib/payload/tddium_payload.rb#L52
* https://github.com/pivotal/projectmonitor/blob/5d8a5bbf3b13e8d4e68d61ff1cde12c92e933e40/lib/payload/cruise_control_xml_payload.rb#L48
* https://github.com/pivotal/projectmonitor/blob/5d8a5bbf3b13e8d4e68d61ff1cde12c92e933e40/lib/payload/jenkins_xml_payload.rb#L55
* https://github.com/pivotal/projectmonitor/blob/5d8a5bbf3b13e8d4e68d61ff1cde12c92e933e40/lib/payload/legacy_team_city_xml_payload.rb#L38

Payloads with the correct value used for published_at:
* https://github.com/pivotal/projectmonitor/blob/master/lib/payload/team_city_xml_payload.rb#L42

We think we have fixed the immediate problem with regards to Travis CI builds, but would like some more context on how to go about fixing this across all types of project.