Probable fix  of pmix_get_version.sh generation.

I've found the reason of aoutogen.sh error I am experiencing:
$ ./pmix_sources_prepare.sh
==> Generating pmix_get_version.sh
autom4te: pmix_get_version.m4sh: no such file or directory

My /bin/sh doesn't support pushd/popd in default installation:
$ sh
$ pushd
sh: 1: pushd: not found

My fix assumes that we can use bash. I am not sure it is portable. Maybe we need to perform some checks of whether or not pushd is supported.