BLD: add SITE_PKG_DIR back in and fix numpy 1.6.1 install
closes #4142 .

I pushed the same SITE_PKG_DIR fix a few minutes ago, and just cherry picked the lxml
fix a moment ago as well.
 where is numpy 1.7.1 being installed?

https://travis-ci.org/cpcloud/pandas/jobs/8785597

there's nothing in the travis docs about numpy being already installed

[these](http://about.travis-ci.org/docs/user/languages/python/#Pre-installed-packages) are the only pre-installed packages.
 i think that the other deps that rely on np need another wheel compiled against 1.6.1 if we want to use it in travis
 @y-p should i close? we can move the discussion over to #4142. see my previous comment
 ok.
