Fix docs on RTD

Failing because this:

```
Collecting Shapely>=1.2.13 (from -r doc-requirements.txt (line 4))
Downloading Shapely-1.5.13.tar.gz (176kB)
Complete output from command python setup.py egg_info:
Failed `CDLL(libgeos_c.so.1)`
Failed `CDLL(libgeos_c.so)`
Traceback (most recent call last):
File "", line 20, in
File "/tmp/pip-build-wOBjbc/Shapely/setup.py", line 38, in
from shapely._buildcfg import geos_version_string, geos_version, \
File "shapely/_buildcfg.py", line 167, in
fallbacks=['libgeos_c.so.1', 'libgeos_c.so'])
File "shapely/_buildcfg.py", line 161, in load_dll
libname, fallbacks or []))
OSError: Could not find library geos_c or load any of its variants ['libgeos_c.so.1', 'libgeos_c.so']

```

Check this:

http://read-the-docs.readthedocs.org/en/latest/faq.html#i-get-import-errors-on-libraries-that-depend-on-c-modules

Start with doc-requirements.txt and the linked Read the Docs FAQ about import errors from C-dependent libraries. Reproduce the Read the Docs build and determine the documented dependency or configuration change needed for Shapely to install. Done means the documentation build completes without the shown GEOS library error.

Written by the indexing model from the issue text.