travis: install composer-asset-plugin

fixes composer error:
- yiisoft/yii2 2.[...] requires bower-asset/jquery 2.2.*@stable | 2.1.*@stable | 1.11.*@stable | 1.12.*@stable -> no matching package found.
...

.travis.yml

@@ -7,6 +7,8 @@ php:
   - '7.0'
   - nightly
 
-install: composer install
+install:
+- composer global require "fxp/composer-asset-plugin:^1.3.1"
+- composer install
 
 script: phpunit --verbose