Install bash completions in /usr/share

With a classic cmake install prefix '/usr',
bash completions were installed in
/usr/etc/bash_completion.d/
which even before bash-completion 1.9 was incorrect.

Since v1.9 all completions are to be installed in
/usr/share/bash-completion/completions
See:
http://anonscm.debian.org/cgit/bash-completion/bash-completion.git/tree/CHANGES#n324

Change the completions install path accordingly