[V3] Bump version to 3.0.0b15 (#1720)

Author: palmtree5

redbot/core/__init__.py

@@ -37,5 +37,5 @@ def to_json(self):
         return [self.major, self.minor, self.micro, self.releaselevel, self.serial]
 
 
-__version__ = "3.0.0b14"
-version_info = VersionInfo(3, 0, 0, "beta", 14)
+__version__ = "3.0.0b15"
+version_info = VersionInfo(3, 0, 0, "beta", 15)