Downgrade to Ubuntu-16.04 for MRI
Downgrade to Ubuntu-16.04 for MRI

This is a tradeoff between having an up-to-date build environment
and as much as possible backward compatiblity for the generated binaries.

See the context here: https://github.com/rake-compiler/rake-compiler-dock/issues/25

Fixes #25

larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,larskanis/rake-compiler-dock,larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock