travis: Change language from python to generic

Travis's python virtualenv setup does not help too much in our case
because we use both python2 and python3 while travis will setup the
virtualenv for only one at a time.

.travis.yml

@@ -29,8 +29,7 @@ install:
     - "sudo `which python2` -m pip install --upgrade $PY_MODS"
     - which python
     - which python2
-language: python
-python: "3.6"
+language: generic
 matrix:
     include:
         - os: linux
@@ -48,7 +47,6 @@ matrix:
                 python3-setuptools
                 python3-tk
         - os: osx
-          language: generic
 script:
     - export TIDYALL_DATA_DIR="$HOME/tidyall_d"
     - bash -x scripts/travis-ci-build