Fixed check-procs.rb path

Default Sensu check config contain old check-procs.rb path
in run.sh script. Updated to match with latest version of
community plugins directory structure.
Fixed Dockerfile

Dockerfile were using wget to download apt-key.
wget command were missing so added wget package installation.
Fixed ruby -command missing from PATH

By default Sensu were using Ruby found from PATH, but
Docker image doesn't have Ruby installed.
Changed Dockerfile so that it configures Sensu to use embedded Ruby
and also add the embedded Ruby to PATH so agent can execute
checks.
Merge pull request #4 from eaapa/master

Multiple small fixes