nokogiri should compile with system libs

As of nokogiri 1.6.0, they default to using the vendored libxml2 libs.
This is problematic for two reasons. First, with the way the heroku
build environment works,
https://github.com/sparklemotion/nokogiri/issues/923. This means it
won't link nokogiri.so properly since it's dependent on hardcoded paths
that don't exist during runtime. Second, compiling libxml2 and friends
is unnecessary since we have them already setup. We should skip this to
speed up deploys.
Merge pull request #124 from heroku/nokogiri

nokogiri should compile with system libs