Revert "upgrade pip"

This reverts commit f870e72.

Author: blueyed

.travis.yml

@@ -19,7 +19,6 @@ jobs:
 install:
   - |
     if [ "$VIM_VERSION" = 'installed' ]; then
-      pip install -U pip
       pip install --user flake8 vim-vint
     else
       bash scripts/install-vim.sh