Updated Sphinx configuration (#11165)

.readthedocs.yml

+version: 2
+
+sphinx:
+  configuration: lib/spack/docs/conf.py
+
+python:
+  version: 3.7
+  install:
+    - requirements: lib/spack/docs/requirements.txt

.travis.yml

@@ -73,7 +73,7 @@ jobs:
           - r-base-core
           - r-base-dev
 
-    - python: '3.6'
+    - python: '3.7'
       sudo: required
       os: linux
       language: python

lib/spack/docs/requirements.txt

 # These dependencies should be installed using pip in order
 # to build the documentation.
 
-sphinx==1.7.0
-sphinxcontrib-programoutput
-sphinx-rtd-theme
+sphinx==2.0.1
+sphinxcontrib-programoutput==0.14
+sphinx-rtd-theme==0.4.3