chore(ci): Remove PHANTOMJS_CDNURL, nit

Seems like PHANTOMJS_CDNURL is not needed anymore now that PhantomJS support is [depracated](https://github.com/mochajs/mocha/blob/6a796cbbcd6c9f805e482c424327c82ed0398dbf/CHANGELOG.md#compatibility).

Please ignore if I am wrong.

.travis.yml

@@ -1,15 +1,9 @@
-# trust dist provides a modern build chain (as opposed to 'precise' dist)
+# trusty dist provides a modern build chain (as opposed to 'precise' dist)
 # which absolves us from having to install compilers and stuff
 dist: trusty
 
 language: node_js
 
-env:
-  global:
-    # phantomjs hosts binaries @ bitbucket, which has fairly restrictive
-    # rate-limiting.  pull it from this sketchy site in China instead.
-    - PHANTOMJS_CDNURL='https://cnpmjs.org/downloads'
-
 matrix:
   fast_finish: true
   include: