Bump APScheduler to >=3.0

This patch upgrades zuul to support APScheduler 3.0. For the most
part, 3.0 was a rewrite but our changes seem to be limited.

Change-Id: I0c66b5998122c3f59ed06e3e7b3ab3199f94f478
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/requirements.txt b/requirements.txt
index 6318a59..8da177a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -12,7 +12,7 @@
 statsd>=1.0.0,<3.0
 voluptuous>=0.7
 gear>=0.5.7,<1.0.0
-apscheduler>=2.1.1,<3.0
+apscheduler>=3.0
 PrettyTable>=0.6,<0.8
 babel>=1.0
 six>=1.6.0