Revert "prefer python3"

This reverts commit b124b18.

Author: blueyed

.travis.yml

@@ -15,7 +15,7 @@ jobs:
 install:
   - |
     if [ "$VIM_VERSION" = 'installed' ]; then
-      python3 -m pip install --user flake8 vim-vint
+      pip install --user flake8 vim-vint
     else
       bash scripts/install-vim.sh
       export PATH=$HOME/vim/bin:$PATH