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%' : '' })

- `[ -f ~/.fzf.${shell} ] && source ~/.fzf.${shell}"`
- `source ~/.fzf.${shell}"`

So only check for file name

Related #95.

Follow symlinks when using bash autocompletion.

Remove `1` from --nth option. With the change you can no more use `$`
anchor to match the tail of a command index. But it makes search
around 15% faster.

    jg@jg:~> time cat history | fzf +s -n..,1,2.. -f fzf > /dev/nul
    real    0m2.929s
    user    0m2.766s
    sys     0m0.154s

    jg@jg:~> time cat history | fzf +s -n2..,.. -f fzf > /dev/null
    real    0m2.535s
    user    0m2.422s
    sys     0m0.112s

Fixed escaping bug of the previous commit

This reverts commit cc9938d4.