[.travis] Use composer for HHVM

This fixes the HHVM build failing because pear doesn't exist in HHVM.

.travis.yml

@@ -3,12 +3,20 @@ sudo: false
 language: php
 
 install:
-  - pear channel-update pear.php.net
-  - pear install PHP_CodeSniffer
+  - if [[ $TRAVIS_PHP_VERSION == "hhvm" ]]; then
+      composer global require squizlabs/PHP_CodeSniffer;
+    else
+      pear channel-update pear.php.net;
+      pear install PHP_CodeSniffer;
+    fi
 
 script:
   - phpenv rehash
-  - phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p
+  - if [[ $TRAVIS_PHP_VERSION == "hhvm" ]]; then
+      /home/travis/.composer/vendor/bin/phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p;
+    else
+      phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p;
+    fi
 
 matrix:
   fast_finish: true