Snap builders can't pull files because of proxy error 407

[Olivier - interfaSys]

I'm not sure if this is due to the release of Ubuntu 16.04, but all armhf and amd64 snap builds I have just tried fail because of a proxy error when trying to clone Github repositories.

Here is one example output

```
Cloning into '/build/owncloud-snap/parts/apache/src'...
fatal: unable to access 'https://github.com/kyrofa/owncloud.git/': Received HTTP code 407 from proxy after CONNECT
Command '['git', 'clone', '--depth', '1', '--recursive', '--branch',
'9.0.1', 'https://github.com/kyrofa/owncloud.git', '/build/owncloud-
snap/parts/apache/src']' returned non-zero exit status 128
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-675/chroot-autobuild', 'linux32', '/bin/sh', '-c', 'cd /build/owncloud-snap && 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
```

[Olivier - interfaSys (olivier-interfasys)]

Happens with regular URLs as well and seems to affect mostly the arm64 builders

[Olivier - interfaSys (olivier-interfasys)]

Still happening today, on amd64, armhf and i386

[Colin Watson (cjwatson)]

We've bumped the proxy token timeout to an hour, so this should be behaving itself much better now.