Use export for Travis variable for convenience

This way, we don't need to set the environment variable in the
`trigger_build.sh` script.

.travis.yml

@@ -9,11 +9,10 @@ services:
   - docker
 
 env:
-  global:
-    - DOCKER_COMPOSE_VERSION=1.23.2
-    - COMMON_BRANCH=master
+  - COMMON_BRANCH=master
 
 before_install:
+  - export DOCKER_COMPOSE_VERSION=1.23.2
   - sudo rm /usr/local/bin/docker-compose
   - curl -L https://github.com/docker/compose/releases/download/${DOCKER_COMPOSE_VERSION}/docker-compose-`uname -s`-`uname -m` > docker-compose
   - chmod +x docker-compose

scripts/trigger_build.sh

@@ -8,10 +8,7 @@ body=$(cat<< EOF
    "config": {
      "merge_mode": "deep_merge",
      "env": {
-      "global": {
-        "DOCKER_COMPOSE_VERSION":"1.23.2",
-        "COMMON_BRANCH": "$TRAVIS_BRANCH"
-       }
+       "COMMON_BRANCH": "$TRAVIS_BRANCH"
      }
     }
   }