Fix wheel script

Author: RazerM

Diff for: .appveyor/install_numpy_scipy.py

@@ -31,7 +31,10 @@ def download_wheel(name, version):
     filename = '{}-{}-{}-none-{}.whl'.format(name, version, py, arch)
 
     directory = 'astrodynamics-wheels'
-    os.mkdir(directory)
+    try:
+        os.mkdir(directory)
+    except OSError:
+        pass
 
     filepath = os.path.join(directory, filename)
     url = base_url + filename