Update to elm 0.19

[DanielCardonaRojas]

Are there plans to upgrade this package to 0.19 ? Or has it been renamed somewhere else ?

[omegaphoenix]

I asked on the Elm Slack and was told "There are also browser restrictions for using watchPosition which makes Geolocation.changes pretty much useless. So you'd probably set up your own ports+handlers for that."

[MrMovl]

Would be nice to put that info in the readme, would save the next few devs some time :)