Add before script section.

This is where we actually build the application to test.

```
# Build the test site (there are many ways of accomplishing this step).
before_script:
  - drush --yes make includes/droplet.make build
  - mkdir -p $CUSTOM_MODULE_PATH
  - git archive $(git rev-parse --abbrev-ref HEAD) | tar -x -C $CUSTOM_MODULE_PATH
  - cd build
  - drush --yes site-install gush --db-url="mysql://$DB_USERNAME@127.0.0.1/$DATABASE"

```