use mock objects to fix build issues on readthedocs

docs/conf.py

@@ -13,10 +13,18 @@
 # serve to show the default.
 
 import sys
-import os
+from mock import Mock as MagicMock
 
-import sphinx_rtd_theme
+class Mock(MagicMock):
+    @classmethod
+    def __getattr__(cls, name):
+            return Mock()
+
+MOCK_MODULES = ['numpy', 'rasterio', 'scipy', 'scikit-image', 'homura', 'boto']
+sys.modules.update((mod_name, Mock()) for mod_name in MOCK_MODULES)
 
+import os
+import sphinx_rtd_theme
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the