Ubuntu build instructions

[Nir Lisker]

Hi,

I'm trying to build OpenJFX on Ubuntu 18.04 and it seems that the
instructions are out of date [1]. I'm getting an error that says that
libgstreamer0.10-dev and libgstreamer-plugins-base0.10-dev can't be found
when executing sudo apt-get install.

Can the instructions be updated?

- Nir

[1]
https://wiki.openjdk.java.net/display/OpenJFX/Building+OpenJFX#BuildingOpenJFX-Ubuntu16.04