Tab-expand environment variables at bash prompt.

This behavior was the default in Bash 3, but it changed in 4.  In
4.2.29, a new option was added specifically to enable the old behavior.

As of this writing, Ubuntu defaults to Bash 4.2, and thus needs the
option set for sane behavior, whereas OS X still defaults to Bash 3.2,
in which the option is not even supported.  It is thus necessary to
check the Bash version in the script.  Note that only the major version
is checked; if you are running Bash 4.1, you may get a warning every
time you start a Bash shell unless you remove the direxpand logic from
bashrc.