Change tshark version check for compatibility

The version string changed in tshark 2.0.2 in Ubuntu 16.
Perhaps we should just check for the version string itself
to be more robust but this is probably OK for now.

mininet/test/test_walkthrough.py

@@ -16,8 +16,8 @@
 def tsharkVersion():
     "Return tshark version"
     versionStr = quietRun( 'tshark -v' )
-    versionMatch = re.findall( r'TShark \d+.\d+.\d+', versionStr )[0]
-    return versionMatch.split()[ 1 ]
+    versionMatch = re.findall( r'TShark[^\d]*(\d+.\d+.\d+)', versionStr )
+    return versionMatch[ 0 ]
 
 # pylint doesn't understand pexpect.match, unfortunately!
 # pylint:disable=maybe-no-member