Ensure test suite runs on PHP 7

sfgeorge · sfgeorge · commit 147a11c0abf0 · 2017-04-04T08:23:12.000-05:00
Prevent this:

```
$ phpunit
PHPUnit 6.0.9 by Sebastian Bergmann and contributors.

Time: 86 ms, Memory: 10.00MB

No tests executed!

The command "phpunit" exited with 0.
```
diff --git a/.travis.yml b/.travis.yml
@@ -8,7 +8,7 @@ php:
   - hhvm
 
 install:
-  - composer install
+  - composer --dev install
 
 script:
-  - phpunit
+  - ./vendor/bin/phpunit
