Merge branch 'bug32240_32242_035' into bug32240_32242_040

Merge:
* libzstd-dev from ticket32242_035
* shellcheck from maint-0.4.0

.travis.yml

@@ -115,8 +115,7 @@ addons:
       - libnss3-dev
       - libscrypt-dev
       - libseccomp-dev
-      ## zstd doesn't exist in Ubuntu Trusty
-      #- libzstd
+      - libzstd-dev
       - shellcheck
       ## Conditional build dependencies
       ## Always installed, so we don't need sudo

changes/ticket32242

+  o Testing (continuous integration):
+    - Use zstd in our Travis Linux builds. Closes ticket 32242.