bootstrap: apt was mangling stdin

When executed "cat bootstrap.sh | bash", apt-get mangled stdin. The script would terminate at the end of the if block containing apt-get (that seems to be as much as bash read from the pipe) and the remainder of the script was output to the console. This was very weird.

Ensuring that apt-get and git have their stdins redirected from /dev/null seems to fix the problem.

see #224