Reconcile python interpreter names

Our shebangs use `python3`, which is the desired name on Linux (where
`python` is still Python 2). But on Windows, Choco's Python only provides a
`python3.exe` executable. Our build scripts deal with this, but we need to
cope when invoking a Python script from Travis itself.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/.travis.yml b/.travis.yml
index 08d7dfb..cdb68ac 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -39,6 +39,7 @@
       env:
         # Add the directory where the Choco packages go
         - PATH=/c/Python35:/c/Python35/Scripts:$PATH
+        - PYTHON=python.exe
       script:
         - type perl; perl --version
         - type python; python --version
@@ -56,7 +57,7 @@
     - secure: "FrI5d2s+ckckC17T66c8jm2jV6i2DkBPU5nyWzwbedjmEBeocREfQLd/x8yKpPzLDz7ghOvr+/GQvsPPn0dVkGlNzm3Q+hGHc/ujnASuUtGrcuMM+0ALnJ3k4rFr9xEvjJeWb4SmhJO5UCAZYvTItW4k7+bj9L+R6lt3TzQbXzg="
 
 install:
-  - scripts/min_requirements.py
+  - $PYTHON scripts/min_requirements.py
 
 addons:
   apt: