vim-vint: use prereleases

Author: blueyed

.travis.yml

@@ -19,7 +19,8 @@ jobs:
 install:
   - |
     if [ "$VIM_VERSION" = 'installed' ]; then
-      pip install --user flake8 vim-vint
+      pip install --user flake8
+      pip install --user --pre vim-vint
     else
       bash scripts/install-vim.sh
       export PATH=$HOME/vim/bin:$PATH