Install packages first to satisfy sbt deps

The sbt package depends on java6-runtime-headless.
Update package cache before fetching
Update marathon submodule to 0.7.5
Git ignore toor asset directory
Add support for centos7 package builds

The centos7 target is based on the fedora target and will produce
uniquely named package names (by appending to $PKG_REL the distro name).
Eventually, we want unique package names for all distro packages. This
is a first step in that direction.
Add EL7 info to README
Enable marathon service by default