Fix continuous integration of pull requests

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