Bundled ElixirLS no longer compatible with Elixir 1.6

[amcvitty]

This is a kind of "Did you mean to break that?" issue, rather than a "This is broken, please fix it!" issue.

The README for this project says that the version of elixir required is 1.6, but I'm seeing an error that the bundled ElixirLS is calling the Mix.Dep.clear_cached/0 function, which first appeared in Elixir 1.7.0

I suggest you update the README accordingly, or if compatibility is important, figure out a different way of doing this

(UndefinedFunctionError)
(mix) Mix.Dep.clear_cached()
(language_server) lib/language_server/build.ex:17: anonymous fn/2 in ElixirLS.LanguageServer.Build.build/3

Compare:
https://github.com/elixir-lang/elixir/blob/v1.6.6/lib/mix/lib/mix/dep.ex
https://github.com/elixir-lang/elixir/blob/v1.7.0/lib/mix/lib/mix/dep.ex