allow compatibility with RHEL7 and systemd

Allow using this module on RHEL/CentOS7, which uses systemd. Without this fix, Puppet will output:

```
Sun Jul 06 23:55:06 -0700 2014 /Stage[main]/Mesos::Slave/Mesos::Service[slave]/Service[mesos-slave] (err): Provider upstart is not functional on this host
```

By limiting the use of upstart as Service provider to os_maj_version < 7, this module then works on RHEL7/CentOS7.