Bump minimum required version of numpy (#594)

It appears that the newly added pulse simulator has an implicit
requirement on having a newer version of numpy installed. We should
encode this minimum in the aer package requirements so people don't
accidently install aer with a version of numpy too old to work.

Fixes #592

setup.py

@@ -22,7 +22,7 @@
 import setuptools
 
 requirements = [
-    'numpy>=1.13',
+    'numpy>=1.16.3',
     'scipy>=1.0',
     'cython>=0.27.1',
     'pybind11>=2.4'  # This isn't really an install requirement,