Use vim-testbed to run Vader tests on Travis

test/example.vader

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+Given (Hello):
+  Hello
+
+Do (yyp):
+  yyp
+
+Expect (Hello\nHello):
+  Hello
+  Hello

test/vimrc

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+" Load builtin plugins
+" We need this because run_vim.sh sets -i NONE
+set rtp=/home/vim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,/home/vim/after
+set rtp+=/vader
+
+" The following is just an example
+filetype plugin indent on
+syntax on
+set nocompatible
+set tabstop=4
+set softtabstop=4
+set shiftwidth=4
+set expandtab
+set backspace=2
+set nofoldenable
+set foldmethod=syntax
+set foldlevelstart=10
+set foldnestmax=10
+set ttimeoutlen=0
+
+let mapleader=','