DOC: add readthedocs yaml config file + use conda for doc deps

doc/environment.yml

@@ -0,0 +1,16 @@
+name: geopandas_docs
+channels:
+- conda-forge
+dependencies:
+- python=3.4
+- pandas
+- shapely
+- fiona
+- pyproj
+- six
+- geopy
+- matplotlib
+- descartes
+- sphinx
+- sphinx_rtd_theme
+- ipython

readthedocs.yml

@@ -0,0 +1,5 @@
+conda:
+    file: doc/environment.yml
+python:
+    version: 3
+    setup_py_install: true

requirements_rtd.txt

@@ -1,2 +0,0 @@
-sphinx_rtd_theme
-ipython