Fix continuous integration of pull requests - Travis doesn't set the $encrypted_* variables for PRs, so disable GPG signing when building a PR (artifacts aren't deployed for PRs anyhow, and even if they were, I wouldn't want them to be signed, as they may contain unvetted code.) - Take advantage of the new -d option in buildljt, which allows for building from an existing Git clone directory. This eliminates the need to rename and restore .git/shallow, allows the official build scripts to work properly when building PRs, and prevents 'git clone' being invoked twice in CI builds. Refer to #217