Update prometheus mmap client library to fix performance issues when rendering many metrics.

Gemfile

@@ -287,7 +287,7 @@ group :metrics do
   gem 'influxdb', '~> 0.2', require: false
   # Prometheus
-  gem 'prometheus-client-mmap', '~>0.7.0.beta11'
+  gem 'prometheus-client-mmap', '~>0.7.0.beta12'
   gem 'raindrops', '~> 0.18'
 end

Gemfile.lock

@@ -619,7 +619,7 @@ GEM
       parser
       unparser
     procto (0.0.3)
-    prometheus-client-mmap (0.7.0.beta11)
+    prometheus-client-mmap (0.7.0.beta12)
       mmap2 (~> 2.2, >= 2.2.7)
     pry (0.10.4)
       coderay (~> 1.1.0)
@@ -1094,7 +1094,7 @@ DEPENDENCIES
   pg (~> 0.18.2)
   poltergeist (~> 1.9.0)
   premailer-rails (~> 1.9.7)
-  prometheus-client-mmap (~> 0.7.0.beta11)
+  prometheus-client-mmap (~> 0.7.0.beta12)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
   rack-attack (~> 4.4.1)