Configure Composer with a GitHub API token to avoid build failures due to rate limiting.

.travis.yml

@@ -45,6 +45,9 @@ before_install:
 
   # Add the ORCA repository.
   - composer config --global repositories.orca vcs "https://github.com/acquia/orca"
+  # Use a personal access token to authenticate with GitHub, to avoid being
+  # rate-limited. The token is configured in the Travis CI repository settings.
+  - composer config --global github-oauth.github.com $GITHUB_API_TOKEN
 
 install:
   - composer create-project --stability dev --no-dev acquia/orca $ORCA