Move all variables to .ci scripts

[tbekolay]

In some cases, like the Python version, configuration has to be specified in .travis.yml. However, in other scripts, some variables are defined in .travis.yml only to be used in a shell script. We should move those variables to the shell script itself so that it can be more easily run independently.

That's the general idea; the specific instance that brought this up was the TEST_ARGS variable, which wasn't defined when I was doing .ci/test-nengo.sh script, so I had to manually export it before running that script.