add dialyzer and fix type errors

[flaviogrossi]

• fixed one typo in start_link type spec
• removed validate_child/1 catchall clause, since it is not reachable because its input is fixed to Supervisor.child_spec/2 return value

Maybe the catchall clause can be kept despite the error and instead a dialyzer ignore can be used just to be explicit about the intended function behaviour, but I think it's better to remove dead code anyway.

dialyzer report before the fix:

lib/pogo/dynamic_supervisor.ex:33:unknown_type
Unknown type: Supervised.name/0.
________________________________________________________________________________
lib/pogo/dynamic_supervisor.ex:450:pattern_match_cov
The pattern
variable_other

can never match, because previous clauses completely cover the type
%{
  :id => _,
  :start => {atom(), atom(), [any()]},
  :modules => :dynamic | [atom()],
  :restart => :permanent | :temporary | :transient,
  :shutdown => :brutal_kill | :infinity | pos_integer(),
  :type => :supervisor | :worker
}.