Updates the Math submodule to 6723f81.

stan-buildbot · stan-buildbot · commit d5d7d215c765 · 2019-10-25T11:10:09.000-04:00
diff --git a/lib/stan_math b/lib/stan_math
@@ -1 +1 @@
-Subproject commit 6455b691681c11e11c3c6ecc6de39a9cf5c3c206
+Subproject commit 6723f81778f3ba939a95908c70d1514bfd322713
