Update .travis.yaml to include the 3 latest elixir versions (#414)

* Update .travis.yaml to include the 3 latest elixir versions

* Avoid using `super` in GenServer callbacks

Using `super` in GenServer callbacks was deprecated in elixir 1.7,
to avoid compilation warnings, this commit replaces all calls to
`super` with an explicit return

.travis.yml

@@ -1,8 +1,8 @@
 language: elixir
 elixir:
-  - 1.4.5
-  - 1.5.3
-  - 1.6.4
+  - 1.6.6
+  - 1.7.4
+  - 1.8.1
 otp_release:
   - 20.3
 env:

lib/wallaby/driver/process_workspace/server.ex

@@ -22,11 +22,11 @@ defmodule Wallaby.Driver.ProcessWorkspace.Server do
     File.rm_rf(workspace_path)
     {:stop, :normal, state}
   end
-  def handle_info(msg, state), do: super(msg, state)
+  def handle_info(_, state), do: {:noreply, state}
 
   @impl GenServer
   def terminate(:shutdown, %{workspace_path: workspace_path}) do
     File.rm_rf(workspace_path)
   end
-  def terminate(reason, state), do: super(reason, state)
+  def terminate(_, _), do: :ok
 end

lib/wallaby/phantom/server.ex

@@ -85,7 +85,7 @@ defmodule Wallaby.Phantom.Server do
   def handle_info({port, {:exit_status, status}}, %ServerState{wrapper_script_port: port} = state) do
     {:stop, {:exit_status, status}, state}
   end
-  def handle_info(msg, state), do: super(msg, state)
+  def handle_info(_, state), do: {:noreply, state}
 
   @impl GenServer
   def terminate(_reason, %ServerState{wrapper_script_port: wrapper_script_port, wrapper_script_os_pid: wrapper_script_os_pid}) do