Build with Ubuntu LTS instead of Debian Stable.

Debian has served us well, but byuu would like to start using C++17 features
which generally requires GCC7. Debian Stable only has GCC6 right now, while
Ubuntu LTS has the required version, so that should get things going again.
4 jobs for master in 14 minutes and 50 seconds
Status Job ID Name Coverage
  Test
passed #140942794
bsnes-linux-x86_64-binaries

00:05:36

passed #140942796
bsnes-windows-x86_64-binaries

00:06:39

passed #140942793
higan-linux-x86_64-binaries

00:09:27

passed #140942795
higan-windows-x86_64-binaries

00:14:50