IJulia!

This installs Julia into the tmpnb image. IJulia won't be setup until https://github.com/JuliaLang/IJulia.jl/pull/210 is more finalized. Right now it seems like the kernel setup isn't getting initialized.

Noticed a problem when building the R kernel too, related to formatting of `install_github`. Fixed that here too.
do you need to get the kernelspec branch?