Check out a common branch through environment var

This will be useful if we trigger the build through the Travis CI API
where we can alter the environment variables in the request.

.travis.yml

@@ -9,7 +9,9 @@ services:
   - docker
 
 env:
-  - DOCKER_COMPOSE_VERSION=1.23.2
+  global:
+    - DOCKER_COMPOSE_VERSION=1.23.2
+    - COMMON_BRANCH=master
 
 before_install:
   - sudo rm /usr/local/bin/docker-compose
@@ -18,6 +20,7 @@ before_install:
   - sudo mv docker-compose /usr/local/bin
 
 install:
+  - git submodule foreach "git checkout $COMMON_BRANCH || echo 'Branch \`$COMMON_BRANCH\` does not exist, falling back to master.'"
   - docker-compose -f Nitro-Backend/docker-compose.yml up -d
   - docker-compose -f Nitro-Web/docker-compose.yml up -d
   - yarn global add cypress wait-on

scripts/trigger.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+body=$(cat<< EOF
+{
+ "request": {
+   "branch":"e2e",
+   "message": "Triggered by \`$TRAVIS_REPO_SLUG\` on \`$TRAVIS_BRANCH\`",
+   "config": {
+     "merge_mode": "deep_merge",
+     "env": {
+      "global": {
+        "DOCKER_COMPOSE_VERSION":"1.23.2",
+        "COMMON_BRANCH": "$TRAVIS_BRANCH"
+       }
+     }
+    }
+  }
+}
+EOF
+)
+
+curl -s -X POST \
+   -H "Content-Type: application/json" \
+   -H "Accept: application/json" \
+   -H "Travis-API-Version: 3" \
+   -H "Authorization: token $TRAVIS_TOKEN" \
+   -d "$body" \
+   https://api.travis-ci.com/repo/Human-Connection%2FHuman-Connection/requests