Do not error on post install script if usermod fails

The usermod command to change the home directory will fail if the
the user is logged in.  We should not fail on this command
as the user will be logged in during upgrades and multiple
application installs.

Fixes: basho/node_package#64
Merge pull request #75 from basho/jem-gh64-rpm-usermod

Do not error on post install script if usermod fails