Use global-requirements.txt in install-packages.sh, if available.

Recent versions of the requirements git repo no longer have a requirements.txt
file, so the 'pip install' command fails. Use global-requirements.txt instead
if that file exists, else fall back to the old behavior.
Merge pull request #4 from steven-collins-omega/global-reqs

Use global-requirements.txt in install-packages.sh, if available.