Do not send TERM signal on Windows

On Windows, the TERM signal is not supported. Just skip it for now.

lib/aruba/platforms/unix_platform.rb

@@ -241,6 +241,10 @@ def which(program, path = ENV["PATH"])
       def builtin_shell_commands
         []
       end
+
+      def term_signal_supported?
+        true
+      end
     end
   end
 end

lib/aruba/platforms/windows_platform.rb

@@ -39,6 +39,10 @@ def which(program, path = ENV["PATH"])
       def builtin_shell_commands
         %w(cd dir echo exit set type)
       end
+
+      def term_signal_supported?
+        false
+      end
     end
   end
 end

lib/aruba/processes/spawn_process.rb

@@ -38,8 +38,10 @@ def stdin
       def stop
         return if @exit_status
 
-        send_signal "TERM"
-        return if poll_for_exit(3)
+        if Aruba.platform.term_signal_supported?
+          send_signal "TERM"
+          return if poll_for_exit(3)
+        end
 
         send_signal "KILL"
         wait