[5.0] Adjusts The Elixir Helper Function to Accommodate a Custom Elixir.json BuildDir Setting

Currently when versioning using elixir one can set a custom build directory as the 2nd argument.  
```
// gulpfile.
elixir(function(mix) {
  mix.sass("application.scss")
     .coffee()
     .phpUnit()
     .version("assets/css/application.css", "public/assets/")
     .routes()
     .events();
});
```
This proposal would help to accommodate this possibility via the elixir helper function seeking out the existence of an elixir.json file with the buildDir setting included.  The implementation was hurried so I'm sure there is a better way to go about integrating this functionality, I just wanted to pass along a passing and failing unit test.  Alternatively we could pass in the override via adding a 2nd argument to the helper function, much like the javascript implementation itself.  

Could you not just go `base_path('elixir.json')`?