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).
Nix spurious escaping.
Merge pull request #38 from craigcitro/idempotent_buildignore

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