Fix unit test suite in the Travis build jobs

This commit fixes a problem in the script that configures the
environment to run the unit tests in the Travis build jobs. The script
was not running `composer install` in the WooCommerce directory, leaving
it installation incomplete and thus the tests were failing.

To fix this problem, this commit adds a call to `composer install` right
after WooCommerce is downloaded in the setup script
(`unit-tests/bin/install.sh`).

unit-tests/bin/install.sh

@@ -181,6 +181,9 @@ install_deps() {
 
 	git clone --depth 1 https://github.com/woocommerce/woocommerce.git
 
+	cd "woocommerce"
+	composer install
+
 	cd "$WP_CORE_DIR"
 	php wp-cli.phar plugin activate woocommerce