Migrate bug bounty URL to bugcrowd

Author: squarejesse

BUG-BOUNTY.md

@@ -6,5 +6,5 @@ can make. We therefore encourage reporting security issues with the code
 contained in this repository.
 
 If you believe you have discovered a security vulnerability, please follow the
-guidelines at https://hackerone.com/square-open-source
+guidelines at https://bugcrowd.com/squareopensource
 

docs/security.md

@@ -17,4 +17,4 @@ can make. We therefore encourage reporting security issues with the code
 contained in this repository.
 
 If you believe you have discovered a security vulnerability, please follow the
-guidelines at https://hackerone.com/square-open-source
+guidelines at https://bugcrowd.com/squareopensource