Pcap.Version - Clarify the exception statement to say that it is like…

…ly that

pcap isn't installed

Previously the statement first implied that the user might have an old pcap version
but that isn't nearly as likely as pcap not being installed.

SharpPcap/Pcap.cs

@@ -64,8 +64,8 @@ public static string Version
                 }
                 catch
                 {
-                    return "pcap version can't be identified, you are either using "+
-                        "an older version, or pcap is not installed.";
+                    return "Pcap version can't be identified. It is likely that pcap is not installed " +
+                        "but you could be using a very old version.";
                 }
             }
         }