Switch travis language to a valid value

Travis doesn't recognize 'vim' and uses 'ruby' instead. Use 'minimal'
for faster turnaround and less noise on tests (smaller image and no
running rake or gem commands).

.travis.yml

@@ -1,4 +1,4 @@
-language: vim
+language: minimal
 
 before_script: |
   git clone https://github.com/junegunn/vader.vim.git vader