Try forcing Travis to deploy with Python 2.7

The latest python tooling has changed and seems to no longer support generating universal wheels. Maybe. It's not clear. But when I locally ``python setup.py sdist bdist_wheel``, I get a universal wheel from python2. But python 3.8 generates an "Instrumental_lib-0.6-cp38-cp38-linux_x86_64.whl", which PyPI refuses to accept. Maybe the newest versions of wheel/setuptools/whatever no longer support generating universal wheels. I have no idea. I just want the pain to stop.

.travis.yml

@@ -20,4 +20,5 @@ deploy:
   distributions: "sdist bdist_wheel"
   on:
     tags: true
-    python: "3.6"
+    python: "2.7"
+    skip_existing: true