Pin rq-scheduler dependency to resolve dependency conflict

The latest version of `flask-rq2` depends on:

* rq-scheduler>=0.8.3
* rq>=0.12,<0.13

Previously, `rq-scheduler` (0.8.3) requires:

* rq>=0.8

A recent release of `rq-scheduler`, 0.9 requires:

* rq>=0.13

`rq-scheduler`'s requirement conflicts directly with `flask-rq2`'s
version requirement of `rq`.

Manully pinning `rq-scheduler` to `>=0.8.3,<0.9` causes `0.8.3` to be
installed, so both dependencies can be satisfied.

Pipfile

@@ -21,6 +21,7 @@ requests = "*"
 apache-libcloud = "*"
 lockfile = "*"
 defusedxml = "*"
+rq-scheduler = ">=0.8.3,<0.9"
 "flask-rq2" = "*"
 
 [dev-packages]