* rename requirements.apt -> requirements-trusty.apt
and requirements.apt.xenial -> requirements-xenial.apt
* improve install_os_dependencies.sh
* remove unused non-python requirements
* rename "install" function to "install_packages" and
"upgrade" function to "upgrade_packages" to solve shellcheck lint errors
* Install python3-dev if use_python2==n
By default, ubuntu-based distros would install python2-dev if you
install python-dev.
python3-dev should be installed explicitly if we're not using python2.
* Add Emanuel Calso to contributors
* Fix python install utility when not using Heroku
Don't try to install `requirements.txt` which doesn't exist unless using Heroku.
* Fix bad absolute paths
* Allow dependency installation from any location
