List dependencies in a text file

List the packages needed to build an RPM on a RHEL-based distro in
**config/dependencies.txt**. Similarly, for Debian-based distros, list
dependencies in **config/dependencies_debian.txt**.

Prior to this, the dependencies had been included in the shell script
`check_dependencies.sh`. (Bad design -- don't include data with
programs!)
Convert the "check dependencies" script to Python

I'm following the sage advice of @mcflugen.

I deleted the old Bash script, **check_dependencies.sh**.
Update .travis.yml and README.md for new Python script