pecl: do not fail if already installed

.travis.yml

@@ -12,7 +12,7 @@ services:
   - mongodb
 
 install:
-  - pecl -q install mongo && echo "extension=mongo.so" >> `php --ini | grep "Loaded Configuration" | sed -e "s|.*:\s*||"`
+  - pecl -s -q install mongo && echo "extension=mongo.so" >> `php --ini | grep "Loaded Configuration" | sed -e "s|.*:\s*||"`
   - composer self-update && composer --version
 #  - composer require satooshi/php-coveralls 0.6.* --dev --prefer-dist
   - echo 'elasticsearch version ' && curl http://localhost:9200/