Declare all jobs as Python

This way we get our chosen Python version everywhere, and pip is available.

Travis doesn't support the python job type on Windows, however, so keep
installing Python manually there.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.travis.yml

@@ -1,5 +1,7 @@
-language: c
-compiler: gcc
+# Declare python as our language. This way we get our chosen Python version,
+# and pip is available. Gcc and clang are available anyway.
+language: python
+python: 3.5
 sudo: false
 cache: ccache
 
@@ -16,8 +18,6 @@ jobs:
           - libnewlib-arm-none-eabi
           - gcc-arm-linux-gnueabi
           - libc6-dev-armel-cross
-      language: python # Needed to get pip for Python 3
-      python: 3.5 # version from Ubuntu 16.04
       script:
         - tests/scripts/all.sh -k 'check_*'
         - tests/scripts/all.sh -k test_default_out_of_box
@@ -30,6 +30,10 @@ jobs:
 
     - name: Windows
       os: windows
+      # The language 'python' is currently unsupported on the
+      # Windows Build Environment. And 'generic' causes the job to get stuck
+      # on "Booting virtual machine".
+      language: c
       before_install:
         - choco install python --version=3.5.4
       env: