There is no snapd/trusty-proposed. Installation fails.

[Tilman Baumann]

This reactive layer tries to explicitly install `snapd/trusty-proposed`.
But this fails in trusty.

There is a suitably recent snapd version in the repo though.

```
root@juju-5bc85b-1-kvm-0:~# apt-cache policy snapd
snapd:
  Installed: 2.29.4.2~14.04
  Candidate: 2.38~14.04
  Version table:
     2.38~14.04 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
     2.37.4~14.04.1 0
        500 http://security.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
 *** 2.29.4.2~14.04 0
        100 /var/lib/dpkg/status
```
The offending line seems to be
https://git.launchpad.net/layer-snap/tree/reactive/snap.py#n252

It seems unnecessary to go to such lengths to force proposed releases. Is this even smart?

As a workaround, I was able to manually install snapd (via trusty-updates) and then the reactive helper passed fine.