Fixed travis build

Author: Sylvain MARIE

.travis.yml

@@ -7,15 +7,15 @@ matrix:
   include:
 #    - python: 2.6
     - python: 2.7
-      PYYAML_VERSION: ""
+      env: PYYAML_VERSION=""
 #    - python: 3.3
 #    - python: 3.4
     - python: 3.5
-      PYYAML_VERSION: "<5"
+      env: PYYAML_VERSION="<5"  # so that we have at least one with the old version
     - python: 3.6
-      PYYAML_VERSION: ""
+      env: PYYAML_VERSION=""
     - python: 3.7
-      PYYAML_VERSION: ""
+      env: PYYAML_VERSION=""
       dist: xenial
       sudo: true