Move doc dependencies to requirements.txt for readthedocs (#4280)

* Move doc dependencies to requirements.txt for readthedocs

* Move sphinx to doc requirements.

.travis.yml

@@ -86,8 +86,7 @@ install:
   - pip install --upgrade setuptools
   - pip install --upgrade codecov
   - pip install --upgrade flake8
-  - if [[ "$TEST_SUITE" == "doc" ]]; then pip install --upgrade sphinx; fi
-  - if [[ "$TEST_SUITE" == "doc" ]]; then pip install --upgrade sphinxcontrib-programoutput; fi
+  - if [[ "$TEST_SUITE" == "doc" ]]; then pip install --upgrade -r lib/spack/docs/requirements.txt; fi
 
 before_script:
   # Need this for the git tests to succeed.

lib/spack/docs/requirements.txt

@@ -0,0 +1,5 @@
+# These dependencies should be installed using pip in order
+# to build the documentation.
+
+sphinx
+sphinxcontrib-programoutput