update default user-agent string (closes #122)

docs/configuration.rst

@@ -203,7 +203,7 @@ extractor.*.user-agent
 ----------------------
 =========== =====
 Type        ``string``
-Default     ``"Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0"``
+Default     ``"Mozilla/5.0 (X11; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0"``
 Description User-Agent header value to be used for HTTP requests.
 
             Note: This option has no effect on `pixiv` and

docs/gallery-dl.conf

@@ -8,7 +8,7 @@
         "proxy": null,
         "skip": true,
         "sleep": 0,
-        "user-agent": "Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0",
+        "user-agent": "Mozilla/5.0 (X11; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0",
 
         "artstation":
         {

gallery_dl/extractor/common.py

@@ -109,8 +109,8 @@ class Extractor():
         """Set additional headers for the 'session' object"""
         self.session.headers["Accept-Language"] = "en-US,en;q=0.5"
         self.session.headers["User-Agent"] = self.config(
-            "user-agent", ("Mozilla/5.0 (X11; Linux x86_64; rv:54.0) "
+            "user-agent", ("Mozilla/5.0 (X11; Linux x86_64; rv:62.0) "
-                           "Gecko/20100101 Firefox/54.0"))
+                           "Gecko/20100101 Firefox/62.0"))
 
     def _set_cookies(self):
         """Populate the session's cookiejar"""