fixed zbuff test

error introduced in https://github.com/facebook/zstd/blame/3b39ce6b527c5cfe395a187c75b77baf5cc7de02/.travis.yml#L192

.travis.yml

@@ -189,7 +189,7 @@ matrix:
     - name: zbuff test
       if: branch = master
       script:
-        - make test-zbuff
+        - make -C tests test-zbuff
 
     - name: Versions Compatibility Test   # 11.5mn
       if: branch = master