Avoid sourcing /etc/bash_completion.d twice

If bash-completion is installed (default on openSUSE 13.2+) it
already handles this directory; it is also careful to skip some
files that are known to be broken (like acroread.sh).

Ref: http://bugzilla.suse.com/show_bug.cgi?id=907873

Signed-off-by: Andrei Borzenkov <arvidjaar@gmail.com>
Merge pull request #21 from arvidjaar/bnc/907873

Avoid sourcing /etc/bash_completion.d twice