See if the addons approach works for zstd.

.travis.yml

@@ -16,8 +16,14 @@ cache:
   directories:
   - ${HOME}/.conan
 
+addons:
+  apt:
+    packages: zstd
+  homebrew:
+    packages: zstd
+
 before_install:
-- if [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then brew install git-lfs; git lfs install; fi
+- if [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then git lfs install; fi
 install:
 - git clone --depth 1 git://github.com/astropy/ci-helpers.git
 - source ci-helpers/travis/setup_conda.sh