liblz4 is needed (?) but not installed

[Carlos Alexandro Becker]

So, goreleaser's builds suddenly started to fail.

Example of a failed build: https://travis-ci.org/goreleaser/goreleaser/builds/339271563

```
Snapping 'mybin' ...
              /snap/snapcraft/1094/usr/bin/mksquashfs: error while loading shared libraries: liblz4.so.1: cannot open shared object file: No such file or directory
```

So, my first try was to install `liblz4-dev`, which fixes the issue.

I'm not very familiar with snapcraft. I tried to find the commit that broke it, but I didn't really know where to look.

Things I know:
- it was working until yesterday, but `--candidate` was broken for more time
- travis uses ubuntu trusty (which maybe is relevant?)

Anyway, I fixed it for my use case here: https://github.com/goreleaser/goreleaser/pull/545

Thanks!

[Kyle Fazzari (kyrofa)]

Thank you for logging this issue! Although you technically logged it first, there's a little more info in bug #1752964 so I'll mark this one as the dupe.