Remove NOKOGIRI_USE_SYSTEM_LIBRARIES from Travis.  #236.
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.

Kirishima21/mastodon,im-in-space/mastodon,vahnj/mastodon,glitch-soc/mastodon,im-in-space/mastodon,vahnj/mastodon,vahnj/mastodon,vahnj/mastodon,im-in-space/mastodon,im-in-space/mastodon,Kirishima21/mastodon,Kirishima21/mastodon,glitch-soc/mastodon,glitch-soc/mastodon,glitch-soc/mastodon,Kirishima21/mastodon,vahnj/mastodon