Upsert

* Bug fixes

* Fix unique constraint detection on pg >9.5.5 (pnomolos https://github.com/seamusabshere/upsert/pull/99)
* Fix Ruby 1.9 tests

* Enhancements

* Use native "upsert" on Postgres 9.5+! (thanks to pnomolos https://github.com/seamusabshere/upsert/pull/79)
* More modern CI tests

* Enhancements

* Test on Ruby 2.3 - thanks Ch4s3 https://github.com/seamusabshere/upsert/pull/70

* Bug fixes

* Stop using Thread.exclusive - thanks hpetru https://github.com/seamusabshere/upsert/pull/67

* Enhancements

* Assume function exists to avoid huge amounts of recreation

* Bug fixes

* Thread safety with Sidekiq! thanks evadne and thbar ! https://github.com/seamusabshere/upsert/pull/47

* Known issues

* speed_spec fails against activerecord-import on mysql... need some advice on properly testing it

* Bug fixes

* Support mysql returning column info as symbols - thanks pnomolos

* Enhancements

* Travis and misc test fixes - thanks raviolicode
* Backwards compat with 1.8 - thanks raviolicode