fzf may not run correctly on some OS even when the binary the platform
is successfully downloaded. The install script is updated to check if
the system has no problem running the executable and fall back to Ruby
version when necessary.

Use travis' new build workers

They boot faster and since we don't use root we can use them.

https://github.com/junegunn/fzf/issues/111#issuecomment-67832143

Reverse the order of fish history

Change `/bin/bash` to `/usr/bin/env bash`

Add --with-nth option

Hashes are unordered on Ruby 1.8

This makes it easier to override FZF command like follows:

    autocmd VimEnter * command! -nargs=? -bang -complete=dir FZF call fzf#run({
          \ 'sink': 'tabe',
          \ 'dir': <q-args>,
          \ 'options': '-m',
          \ 'tmux_height': empty('<bang>') ? '40%' : '' })