Run snapcraft update before attempting a snap build

It'd be good to run

  snapcraft update

before a build. This will update snapcraft with the list of current remote parts.

From https://launchpadlibrarian.net/271441376/buildlog_snap_ubuntu_xenial_amd64_qownnotes_BUILDING.txt.gz

Issue detected while analyzing snapcraft.yaml: Cannot find definition for part 'desktop/qt5'. It may be a remote part, run `snapcraft update` to refresh the remote parts cache
Traceback (most recent call last):
  File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 194, in main
    builder.pull()
  File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 135, in pull
    env=env)
  File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 92, in run_build_command
    self.chroot(["/bin/sh", "-c", command], echo=echo)
  File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 66, in chroot
    "/usr/bin/sudo", "/usr/sbin/chroot", self.chroot_path] + args)
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
CalledProcessError: Command '['/usr/bin/sudo', '/usr/sbin/chroot', '/home/buildd/build-SNAPBUILD-1441/chroot-autobuild', 'linux64', '/bin/sh', '-c', 'cd /build/qownnotes && env LANG=C.UTF-8 https_proxy=https://snap-proxy.launchpad.net:3128 http_proxy=http://snap-proxy.launchpad.net:3128 SNAPCRAFT_LOCAL_SOURCES=1 snapcraft pull']' returned non-zero exit status 1