Fix issue in mix task - undefined function: HashDict.new/1

Ran into an issue running 'mix weber.new' in my environment.

Using
 - Erlang/OTP 17 
 - Elixir 0.13.2-dev


```
mix weber.new ~/Projects/weber_test
** (UndefinedFunctionError) undefined function: HashDict.new/1
    (elixir) HashDict.new([{"path", "~/Projects/weber_test"}, {"projectNamespace", "WeberTest"}, {"projectName", "weber_test"}])
    lib/mix/tasks/weber.ex:60: Mix.Tasks.Weber.New.run/1
    (mix) lib/mix/cli.ex:64: Mix.CLI.run_task/2
    (elixir) src/elixir_lexical.erl:17: :elixir_lexical.run/2
    (elixir) lib/code.ex:303: Code.require_file/2
```