fix ruby 2.7.0 deprecation warning: deprecated Object#=~ is called on…

… FalseClass; it always returns nil

lib/net/ping/external.rb

@@ -66,7 +66,7 @@ def ping(host = @host, count = 1, interval = 1, timeout = @timeout)
                 bool = true  # Success, at least one response.
               end
 
-              if err & err =~ /warning/i
+              if err & (err =~ /warning/i)
                 @warning = err.chomp
               end
             when 2