pin holoviews=1.11 in environment.yml for RTD

Currently, the plots aren't displayed on RTD.

docs/environment.yml

@@ -6,19 +6,16 @@ channels:
 dependencies:
   - python=3.6
   - pip:
-    - sortedcontainers
+    - sortedcollections
     - scipy
-    - holoviews
-    - bokeh==0.13
+    - holoviews==1.11
+    - bokeh
     - plotly
     - ipyparallel
+    - pandas  # until https://github.com/ioam/holoviews/issues/3430 is fixed
     - distributed
-    - ipykernel>=4.8*
-    - jupyter_client>=5.2.2
     - ipywidgets
     - scikit-optimize
-    - nbconvert>=5.4*  # because my jupyter-sphinx branch depends on it
-    - sphinx_rtd_theme
     - git+https://github.com/basnijholt/jupyter-sphinx.git@widgets_execute
     - git+https://github.com/python-adaptive/adaptive.git@master  # temporary solution because jupyter-sphinx doesn't use the adaptive from the repo
     - sphinx_fontawesome