load `tshark` executable returned by `which tshark` about pyshark

[bbayles]

@goretkin, could you try my branch? I think this is a good idea.

[goretkin]

@bbayles thanks! which will return a new-line terminated path, so that needs to be stripped. Also check_output needs to be qualified. Then it works great!

@@ -36,7 +36,7 @@ def get_tshark_path():
     # `which tshark`, common paths.
     else:
         try:
-            which_tshark = check_output(['which', 'tshark']).decode('ascii')
+            which_tshark = subprocess.check_output(['which', 'tshark']).decode('ascii').strip()
         except subprocess.CalledProcessError:
             pass
         else:

[bbayles]

I like @baszoetekouw's method better than mine - I've replaced my method with his, and added it to #58.

@baszoetekouw and @goretkin, I will merge these on Wednesday morning unless @KimiNewt has some objection.

Thanks!