travis: use stable channel for shellcheck snap

Fixes failure with:
```
shellcheck: src/ShellCheck/Analytics.hs:1825:10-27:
No instance nor default method for class operation mappend
```

It's probably not a good idea to use the edge channel for CI
when not required, so just pivot to stable channel.

Signed-off-by: Chris Patterson <chris.patterson@canonical.com>

Author: Chris Patterson

.travis.yml

@@ -24,7 +24,6 @@ jobs:
           - name: core
           - name: core18
           - name: shellcheck
-            channel: edge
           - name: black
             channel: beta
             confinement: devmode