Being explicit about oldest dependency in tox, numpy has to at max what oldest astropy supports

Author: bsipocz

tox.ini

@@ -29,9 +29,12 @@ description = run tests
 deps =
     devastropy: git+https://github.com/astropy/astropy.git#egg=astropy
 
-# TODO: Add more versions to oldestdeps
-    oldestdeps: astropy==3.1
+# TODO: Add more versions to oldestdeps. numpy<1.15 could not be installed
+# in CI, a much newer version was pulled in instead, thus setting
+# minimum numpy to 1.15.
 
+    oldestdeps: astropy==3.1.2
+    oldestdeps: numpy==1.15
     cov: codecov
 
 extras =