default to expert for the lsp

The install instructions included something I missed. It includes a
global install, where the default executable is just `expert`. I'll
default to that and review if I should should add a configuration for a
global config.

Author: Your Name

ale_linters/elixir/expert.vim

@@ -1,7 +1,7 @@
 " Author: Paul Monson <[email protected]>
 " Description: Expert integration (https://github.com/elixir-lang/expert)
 
-call ale#Set('elixir_expert_executable', '/usr/bin/elixir-expert')
+call ale#Set('elixir_expert_executable', 'expert')
 
 call ale#linter#Define('elixir', {
 \   'name': 'expert',

test/linter/test_elixir_expert.vader

@@ -5,7 +5,7 @@ After:
   call ale#assert#TearDownLinterTest()
 
 Execute(should set correct defaults):
-  AssertLinter '/usr/bin/elixir-expert', ale#Escape('/usr/bin/elixir-expert')
+  AssertLinter 'expert', ale#Escape('expert')
 
 Execute(The executable should be configurable):
   let b:ale_elixir_expert_executable = 'foobar'