[mangadex] fix manga extraction (closes #84)

Chapter listings for manga now use
https://mangadex.org/manga/<id>/_/chapters/2/
as URL instead of
https://mangadex.org/manga/<id>/_//2/

test/test_results.py

@@ -21,6 +21,7 @@ TRAVIS_SKIP = {
 
 # temporary issues, etc.
 BROKEN = {
+    "pixiv",  # API requests sometimes fail
 }