refactor(systemd): clean up paths

harcoded paths are not required in shell environments. I've also
removed unnecessary uses of /bin/sh when we only require an
environment variable, since systemd can reference environment
variables through ${}.

ref: http://www.freedesktop.org/software/systemd/man/systemd.service.html#ExecStart=
Merge pull request #888 from deis/systemd-refactor

refactor(systemd): clean up paths