Fix determining gs_path on Linux. Fixes jwdj#66

When running EasyABC for the first time under Linux, it tries to
determine the path to gs from the output of `which gs`. However, in
python3, it returns a bytestring. EasyABC is expecting a str.  Simply
adding .decode() fixes the issue. The unicode() call is a remnant from
python2 and can be removed.

easy_abc.py

@@ -8368,7 +8368,7 @@ def restore_settings(self):
             elif wx.Platform == '__WXGTK__':
                 try:
                     gs_path = subprocess.check_output(["which", "gs"])
-                    settings['gs_path'] = unicode(gs_path[0:-1])
+                    settings['gs_path'] = gs_path[0:-1].decode()
                 except:
                     settings['gs_path'] = ''
             #1.3.6.1 [SS] 2014-01-13