Remove NOKOGIRI_USE_SYSTEM_LIBRARIES from Travis. #236.

Nokogiri linked to the version of libxml2 shipped in Travis' Trusty
image exhibits incorrect behavior when parsing strings resembling

    <p>I <3 oats</p>

In particular, Html2Text.convert (which uses nokogiri) will return "I"
for the above string when it ought to return "I <3 oats".

The version of libxml2 shipped with nokogiri 1.8.1+ exhibits correct
parse behavior for this string, as does the libxml2 present in Ubuntu
Xenial.

.travis.yml

@@ -18,7 +18,6 @@ env:
     - LOCAL_DOMAIN=cb6e6126.ngrok.io
     - LOCAL_HTTPS=true
     - RAILS_ENV=test
-    - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
     - PARALLEL_TEST_PROCESSORS=2
     - "PATH=$HOME:$PATH"