ubuntu install update

I noticed that I needed to install libz-dev on Ubuntu 13.10 before I could build lxml. I got this error:

```
x86_64-linux-gnu-gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/libxml2 -I/home/ubuntu/.virtualenvs/pillbox/build/lxml/src/lxml/includes -I/usr/include/python2.7 -c src/lxml/lxml.etree.c -o build/temp.linux-x86_64-2.7/src/lxml/lxml.etree.o -w

x86_64-linux-gnu-gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-Bsymbolic-functions -Wl,-z,relro -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -D_FORTIFY_SOURCE=2 -g -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security build/temp.linux-x86_64-2.7/src/lxml/lxml.etree.o -lxslt -lexslt -lxml2 -lz -lm -o build/lib.linux-x86_64-2.7/lxml/etree.so

/usr/bin/ld: cannot find -lz

collect2: error: ld returned 1 exit status

error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

----------------------------------------

Cleaning up...

  Removing temporary dir /home/ubuntu/.virtualenvs/pillbox/build...
Command /home/ubuntu/.virtualenvs/pillbox/bin/python -c "import setuptools;__file__='/home/ubuntu/.virtualenvs/pillbox/build/lxml/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-72GjJH-record/install-record.txt --single-version-externally-managed --install-headers /home/ubuntu/.virtualenvs/pillbox/include/site/python2.7 failed with error code 1 in /home/ubuntu/.virtualenvs/pillbox/build/lxml

Exception information:
Traceback (most recent call last):
  File "/home/ubuntu/.virtualenvs/pillbox/local/lib/python2.7/site-packages/pip/basecommand.py", line 134, in main
    status = self.run(options, args)
  File "/home/ubuntu/.virtualenvs/pillbox/local/lib/python2.7/site-packages/pip/commands/install.py", line 241, in run
    requirement_set.install(install_options, global_options, root=options.root_path)
  File "/home/ubuntu/.virtualenvs/pillbox/local/lib/python2.7/site-packages/pip/req.py", line 1298, in install
    requirement.install(install_options, global_options, *args, **kwargs)
  File "/home/ubuntu/.virtualenvs/pillbox/local/lib/python2.7/site-packages/pip/req.py", line 625, in install
    cwd=self.source_dir, filter_stdout=self._filter_install, show_stdout=False)
  File "/home/ubuntu/.virtualenvs/pillbox/local/lib/python2.7/site-packages/pip/util.py", line 670, in call_subprocess
    % (command_desc, proc.returncode, cwd))
InstallationError: Command /home/ubuntu/.virtualenvs/pillbox/bin/python -c "import setuptools;__file__='/home/ubuntu/.virtualenvs/pillbox/build/lxml/setup.py';exec(compile(open(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-72GjJH-record/install-record.txt --single-version-externally-managed --install-headers /home/ubuntu/.virtualenvs/pillbox/include/site/python2.7 failed with error code 1 in /home/ubuntu/.virtualenvs/pillbox/build/lxml
```