Remove all but most recent HHVM 3

As per this announcement (https://hhvm.com/blog/2018/12/17/hhvm-3.30.html), Facebook are deprecating PHP support in HHVM. 3.30 is the last LTS version. As such, I've removed all old versions from the tests. As soon as docopt breaks with 3.30, I will probably bump the major version of docopt.php, remove all vestiges of support for HHVM, and direct users to install the old version if they need to keep using it.

Author: shabbyrobe

.travis.yml

@@ -10,12 +10,7 @@ before_install:
 # hhvm-3.6,: command not found
 matrix:
   include:
-    - { dist: trusty , php: "hhvm-3.3" }
-    - { dist: trusty , php: "hhvm-3.6" }
-    - { dist: trusty , php: "hhvm-3.9" }
-    - { dist: trusty , php: "hhvm-3.12" }
-    - { dist: trusty , php: "hhvm-3.15" }
-    - { dist: trusty , php: "hhvm-3.18" }
+    - { dist: trusty , php: "hhvm-3.30" }
     - { dist: trusty , php: "nightly" }
     - { dist: trusty , php: "7.4snapshot" }
     - { dist: trusty , php: "7.3" }