Add `travis-tool` to `.Rbuildignore` automatically and idempotently.

This is a little on the fancy side, but will be important if we ever want to
push the built packages anywhere (since they'll accidentally pick up
`travis-tool.sh` otherwise).
Tweak a test yet again.
Remove a currently broken test.

Something funny is going on here; I'm filing #41 to track it.
Nix spurious escaping.
More tests in .travis.yml.

Fixes #41.
Test multiple arguments for aptget_install.
Merge pull request #38 from craigcitro/idempotent_buildignore

Add `travis-tool` to `.Rbuildignore` automatically and idempotently.
Merge branch 'master' into ensure_devtools