stop building with hhvm because it stopped working (#146)

it seems a recent version of hhvm did a BC break, builds that have been successful now fail with
```
Fatal error: Uncaught TypeError: Argument 1 passed to spec\Http\Client\Exception\HttpExceptionSpec::let() must implement interface Psr\Http\Message\RequestInterface, PhpSpec\Wrapper\Collaborator given in /home/travis/build/php-http/httplug/spec/Exception/HttpExceptionSpec.php:14
```

Author: dbu

.travis.yml

@@ -27,8 +27,6 @@ matrix:
     include:
         - php: 5.4
           env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE=true TEST_COMMAND="composer test-ci"
-        - php: hhvm
-          dist: trusty
 
 before_install:
     - if [[ $COVERAGE != true && $TRAVIS_PHP_VERSION != hhvm ]]; then phpenv config-rm xdebug.ini || true; fi