Use composer install instead of composer update in Travis (#12947)

.travis.yml

@@ -72,7 +72,7 @@ matrix:
         - phpenv config-rm xdebug.ini || echo "xdebug is not installed"
         - travis_retry composer self-update && composer --version
         - travis_retry composer global require "fxp/composer-asset-plugin:^1.2.0" --no-plugins
-        - travis_retry composer update --prefer-dist --no-interaction
+        - travis_retry composer install --prefer-dist --no-interaction
       before_script:
         - node --version
         - npm --version
@@ -111,7 +111,7 @@ install:
   - travis_retry composer global require "fxp/composer-asset-plugin:^1.2.0" --no-plugins
   - export PATH="$HOME/.composer/vendor/bin:$PATH"
 # core framework:
-  - travis_retry composer update --prefer-dist --no-interaction
+  - travis_retry composer install --prefer-dist --no-interaction
   - tests/data/travis/apc-setup.sh
   - tests/data/travis/memcache-setup.sh
   - tests/data/travis/imagick-setup.sh