Some minor improvments

- Use @garbas fork of [SnipMate](/garbas/vim-snipmate), which is still under maintained.
- Install [YCM](/Valloric/YouCompleteMe) for auto completion.
- Alter SnipMate's trigger key to `<leader>s` for the compatibility with YCM.
- Set how many lines of history VIM has to remember.
- In normal mode, we use `:` much more often than `;` so lets swap them.