changed dpkg -l to dpkg -s for the post-install script so that it will detect the correct package when ran on the monitor server
changed the post-install script to check for the monitor hardening package instead of the meta securedrop-monitor package since securedrop-monitor-hardening can be installed without the meta package securedrop-monitor
Merge pull request #588 from dolanjs/post-install-pkg-detection

Post install pkg detection