* el-get.el: new defun `el-get-update-specific-backend`
  for updating   specific backend recipes.

  Fixes #1042
add recipe for elixir-mix.el
Merge pull request #1413 from tonini/master

add recipe for elixir-mix.el
Reimplement el-get-update-specific-backend as el-get-update-packages-of-type

New definition is more canonical elisp, and uses two new el-get-core
functions "el-get-plist-keys" and "el-get-keyword-name".

Intended to improve #1412.
Merge branch 'el-get-update-specific-backend'