Update .travis.yml

Author: GrahamCampbell

.travis.yml

@@ -4,42 +4,42 @@ jobs:
       dist: trusty
       language: php
       php: hhvm-3.18
-      before_install: composer self-update --2
+      before_install: composer self-update --preview
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: HHVM 3.30
       dist: trusty
       language: php
       php: hhvm-3.30
-      before_install: composer self-update --2
+      before_install: composer self-update --preview
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: PHP 5.3
       dist: precise
       language: php
       php: 5.3
-      before_install: composer self-update --2
+      before_install: composer self-update --preview
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: PHP 5.4
       dist: trusty
       language: php
       php: 5.4
-      before_install: composer self-update --2
+      before_install: composer self-update --preview
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: PHP 5.5.9
       dist: trusty
       language: php
       php: 5.5.9
-      before_install: composer self-update --2
+      before_install: composer self-update --preview
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: PHP 5.5
       dist: trusty
       language: php
       php: 5.5
-      before_install: composer self-update --2
+      before_install: composer self-update --preview
       install: travis_retry composer update -n -o
       script: vendor/bin/phpunit
     - name: PHP 5.6