before_install commands run before r-travis envirnoment is fully set up

[Carl Boettiger]

I'd like the utilities installed by r-travis (e.g. devtools) to be available to commands made in the before_install: section (e.g. devtools::install_version()). Currently, it looks like I have to install devtools manually in before_install. Not a big problem, but a bit of a surprise since r-travis normally installs devtools automatically.