Travis: Use zstd in our Travis Linux builds

Recent Ubuntu versions have zstd >= 1.1, which is an optional tor build
dependency.

Closes ticket 32242.

.travis.yml

@@ -103,8 +103,7 @@ addons:
       - libnss3-dev
       - libscrypt-dev
       - libseccomp-dev
-      ## zstd doesn't exist in Ubuntu Trusty
-      #- libzstd
+      - libzstd-dev
       ## Conditional build dependencies
       ## Always installed, so we don't need sudo
       - asciidoc

changes/ticket32242

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