Require Vim 8 or NeoVim

Now that we're using `ALE`, we're making use of the async features
available in Vim8 and NeoVim. If you try to use these dotfiles on Vim 7,
you'll get a host of errors from ALE.

This change does two things:

1. It writes an error when starting in an unsupported vim version.
2. It skips loading ALE in those versions.

The thinking behind the second change is to leave vim in a more usable
state with minimal annoyance after the initial error.

vimrc

@@ -12,6 +12,8 @@ set incsearch     " do incremental searching
 set laststatus=2  " Always display the status line
 set autowrite     " Automatically :write before running commands
 
+let g:has_async = v:version >= 800 || has('nvim')
+
 " Switch syntax highlighting on, when the terminal has colors
 " Also switch on highlighting the last used search pattern.
 if (&t_Co > 2 || has("gui_running")) && !exists("syntax_on")
@@ -46,12 +48,16 @@ augroup vimrcEx
   autocmd BufRead,BufNewFile .{jscs,jshint,eslint}rc set filetype=json
 
   " ALE linting events
-  set updatetime=1000
-  let g:ale_lint_on_text_changed = 0
-  autocmd CursorHold * call ale#Lint()
-  autocmd CursorHoldI * call ale#Lint()
-  autocmd InsertEnter * call ale#Lint()
-  autocmd InsertLeave * call ale#Lint()
+  if g:has_async
+    set updatetime=1000
+    let g:ale_lint_on_text_changed = 0
+    autocmd CursorHold * call ale#Lint()
+    autocmd CursorHoldI * call ale#Lint()
+    autocmd InsertEnter * call ale#Lint()
+    autocmd InsertLeave * call ale#Lint()
+  else
+    echoerr "The thoughtbot dotfiles require NeoVim or Vim 8"
+  endif
 augroup END
 
 " When the type of shell script is /bin/sh, assume a POSIX-compatible

vimrc.bundles

@@ -56,7 +56,10 @@ Plug 'tpope/vim-rhubarb'
 Plug 'tpope/vim-surround'
 Plug 'vim-ruby/vim-ruby'
 Plug 'vim-scripts/tComment'
-Plug 'w0rp/ale'
+
+if g:has_async
+  Plug 'w0rp/ale'
+endif
 
 if filereadable(expand("~/.vimrc.bundles.local"))
   source ~/.vimrc.bundles.local