Use CI for preview of PRs

[PetarKirov]

Similar to DAutoTest, but simpler to implement: https://github.com/rejectedsoftware/ddox/blob/v0.16.0/travis-ci.sh#L23

[wilzbach]

With our current low load of PRs it's imho easier to just use the GH feature to clone the PR directly from upstream, e.g.

~/.gitconfig

[alias]
pr  = "!f() { git fetch -fu ${2:-upstream} refs/pull/$1/head:pr/$1 && git checkout pr/$1; }; f"

and then:

git pr 5150 dub

It takes usually less than a minute ;-)