Try downgrading version of NumPy used in CI

But not for doctests, because the formatting was so ugly back then.
The `pip install` right before the doctests are run should re-upgrade
to a prettier NumPy.

Author: brandon-rhodes

.travis.yml

@@ -23,7 +23,7 @@ install:
   - "pip install --upgrade pip"
   - "pip install $(echo ../dist/skyfield-*.tar.gz)'[tests]'"
   - "pip install https://github.com/brandon-rhodes/assay/archive/master.zip"
-  - "pip install pandas==0.24.2 unittest2"
+  - "pip install numpy==1.11.3 pandas==0.24.2 unittest2"
   - "python -c 'import pandas'"
 
 script:

skyfield/documentation/design.rst

@@ -180,6 +180,12 @@ the hundreds of ``np.`` prefixes would add only noise.
 As a consequence, Skyfield’s modules themselves simply do a
 ``from`` ``numpy`` ``import`` of any names that they need.
 
+Skyfield strives to support old versions of both Python and of NumPy,
+because many users in industry and government
+cannot upgrade their supporting libraries whenever they want.
+So the unit tests in CI are run against NumPy 1.11.3
+to maintain compatibility with that older version.
+
 Cross products
 ==============