Add Composer dist files to Travis cache

This also removes "--prefer-source", which should result in Composer downloading dist releases by default.

.travis.yml

@@ -10,6 +10,7 @@ addons:
 cache:
   directories:
     - ${HOME}/.cache/pip
+    - ${HOME}/.composer/cache/files
 
 env:
   global:
@@ -51,7 +52,7 @@ before_script:
   - pushd ${MO_PATH} && ${TRAVIS_BUILD_DIR}/.travis/mo.sh configurations/servers/clean.json start && popd
   - pecl install -f mongodb-${DRIVER_VERSION}
   - php --ri mongodb
-  - composer install --dev --no-interaction --prefer-source
+  - composer install --no-interaction --no-progress --no-suggest
   - ulimit -c
   - ulimit -c unlimited -S