Use StackPath's URL for the Bootstrap CDN
What
Changes the Bootstrap CDN from MaxCDN's one with StackPath's one.
Why
As upgraded to Bootstrap 4.3 (!336 (merged)), we should use the latest host of bootstrap CDN at stackpath.bootstrapcdn.com
(4.1 or later). cf. maxcdn.bootstrapcdn.com
(4.0 or before)
Closes #386 (closed)
Edited by Takuya Noguchi