citrain tool needs to pin the silo with higher score than overlay PPA

Otherwise the silo contents do not get upgraded from the version in the overlay PPA anymore:

---
$ apt-cache policy libqt5qml5
libqt5qml5:
  Installed: 5.4.1-1ubuntu6
  Candidate: 5.4.1-1ubuntu6
  Version table:
     5.4.1-1ubuntu7 0
        500 http://ppa.launchpad.net/ci-train-ppa-service/landing-002/ubuntu/ vivid/main armhf Packages
 *** 5.4.1-1ubuntu6 0
       1001 http://ppa.launchpad.net/ci-train-ppa-service/stable-phone-overlay/ubuntu/ vivid/main armhf Packages
        100 /var/lib/dpkg/status
     5.4.1-1ubuntu5 0
        500 http://ports.ubuntu.com/ubuntu-ports/ vivid/main armhf Packages
---