zlib1g-dev is needed to compile lxml

On Debian Jessie the zlib1g-dev package was needed to compile lxml during the pip install. It seems this might affect Ubuntu 10.04+ too (http://stackoverflow.com/a/17378157/862188)
Merge pull request #4 from zertrin/fix-readme-1

Fix README: zlib1g-dev is needed to compile lxml