Fix autoload for phoenix

When dializer isn't a dependency, mix dialyzer recompiles the whole
project because it's not possible to know if this command dialyzer exist
or not until recompilation is done. Then the timestamps of the project
is messed up which results in broken hot-loading. In this case, mix help
dialyzer would return zero which prevents compilation of the whole
project since dialyzer isn't installed, it's help manual doesn't exist.

When dialyzer is a dependency, mix dialyzer would just run the command.
In this case, mix help dialyzer would return 1 which allows mix dialyzer
to run.

ale_linters/elixir/dialyxir.vim

@@ -28,7 +28,7 @@ endfunction
 call ale#linter#Define('elixir', {
 \   'name': 'dialyxir',
 \   'executable': 'mix',
-\   'command': 'mix dialyzer',
+\   'command': 'mix help dialyzer && mix dialyzer',
 \   'callback': 'ale_linters#elixir#dialyxir#Handle',
 \})