add 7.4 and explicit distributions in travis config
add 7.4 and explicit distributions in travis config

Add PHP 7.4 to the travis config, also...

Travis build broke because the default dist changed. To prevent this
from happening in future, this commit changes each version to run on a
specific linux distibution.

It also cleans up travis config warnings while we're in there, but
that's trivial.

Vectorface/SnappyRouter,Vectorface/SnappyRouter