on system that don't hard code bash's path.

For example, BSD's bash lives in /usr/local/bin on freebsd & openbsd.

Patch contributed by Sevan Janiyan (venture37 at gmail)

TEST=didn't break ubuntu users
BUG=none

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@66124 0039d316-1c4b-4281-b951-d872f2087c98