Use reactphp/socket#109 as suggested by @clue for HHVM travis config

Author: WyriHaximus

.travis.yml

@@ -6,9 +6,12 @@ php:
   - 5.5
   - 5.6
   - 7
-  - hhvm
 
+# also test against HHVM, but require "trusty" and ignore errors
 matrix:
+  include:
+    - php: hhvm
+      dist: trusty
   allow_failures:
     - php: hhvm