Add 6502 tests to travis

This pull adds travis tests for the following platforms:

`apple2enh`, `atarixl`, `c64` and `c128`

Currently, travis will clone cc65 from oliverschmidt/cc65 and build it. The problem with this approach is:
* If cc65's master branch breaks, our tests here also break
* It's a bit slow but that's not a huge deal. The entire 6502 test suite takes 6-7 mins on travis

Ideally, we'd be able to simply download a pre-built and tested cc65 bundle. Failing that, we can checkout a certain cc65 tag/branch/hash and build that one instead of master's HEAD

Nevertheless, this works at the moment.