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).
Change `DevtoolsInstall` to `EnsureDevtools`.

This simply checks for `devtools` and installs it if needed. As a nice
by-product, we can also remove the `HAS_DEVTOOLS` environment variable.
experiment with exit status
Make `devtools_install` a noop.
Code review feedback.
Nix spurious escaping.
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