Use "new" branch in 99designs babushka-deps source

This is a massive hack to ensure that when loading a babushka dep
source, if the source is 99designs/babushka-deps on GitHub, the "new"
branch is always used (instead of the usual "master").

This commit will be part of a different branch in this repository,
which will allow this functionality to be opted-in to.
Avoid "broken pipe" errors from shell commands

When executing a shell command, Babushka sets up pipes for each of the
stdin, stdout, and stderr connected to the sub-process. If the process
decides it doesn't need any more data coming into its stdin and closes
it, Babushka should stop writing into it (which would cause a broken
pipe error).

The implementation is to catch such an error and close the pipe if that
happens. Babushka already won't write into the pipe if it's closed on
our end (the write end).