mod_prometheus: Specify the dependency desired version also in git

When ejabberd is running as an OTP release, mix may not be available,
and dependencies are fetched using git.

Author: badlop

mod_prometheus/rebar.config

@@ -1,4 +1,4 @@
 {deps, [
         {quantile_estimator, "1.0.2", {git, "https://github.com/odo/quantile_estimator", {branch, "master"}}},
-        {prometheus, "5.0.0", {git, "https://github.com/prometheus-erl/prometheus.erl", {branch, "master"}}}
+        {prometheus, "5.0.0", {git, "https://github.com/prometheus-erl/prometheus.erl", {tag, "v5.0.0"}}}
        ]}.