Update download page for 2019

[AustinShalit]

We need to update http://first.wpi.edu/FRC/roborio/toolchains/ for 2019.

[JaciBrunning]

The correct command to run is ./gradlew installRoboRioToolchain and ./gradlew installRaspbianToolchain

./gradlew installToolchain is removed and replaced by ./gradlew installRoboRioToolchain

[Lytigas]

Having the 2019 toolchain published at that location would be very useful for my team. Being able to manage the toolchain through apt is very nice. Since we're not using gradle (several other teams also do not, I believe) the gradle command and the 1.1 GB full WPILib download are both a little cumbersome compared to old way. This is, as far as I know, one of the only places that you can get an arm-gcc binaries for Mac and Windows.

Please do this if the infrastructure is already there!

[ThadHouse]

You can use the archives published on this repo in the releases area if you don't want the entire installer.

We dropped the packaged installers because they were difficult to maintain and all required admin permissions. These are all now standalone so you can just extract them and add the bin folders to path.