Update prometheus gem to fix build on RPi

Gemfile

@@ -283,7 +283,7 @@ group :metrics do
   gem 'influxdb', '~> 0.2', require: false
   # Prometheus
-  gem 'prometheus-client-mmap', '~> 0.7.0.beta37'
+  gem 'prometheus-client-mmap', '~> 0.7.0.beta39'
   gem 'raindrops', '~> 0.18'
 end

Gemfile.lock

@@ -625,7 +625,7 @@ GEM
       parser
       unparser
     procto (0.0.3)
-    prometheus-client-mmap (0.7.0.beta37)
+    prometheus-client-mmap (0.7.0.beta39)
       mmap2 (~> 2.2, >= 2.2.9)
     pry (0.10.4)
       coderay (~> 1.1.0)
@@ -1111,7 +1111,7 @@ DEPENDENCIES
   peek-sidekiq (~> 1.0.3)
   pg (~> 0.18.2)
   premailer-rails (~> 1.9.7)
-  prometheus-client-mmap (~> 0.7.0.beta37)
+  prometheus-client-mmap (~> 0.7.0.beta39)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
   rack-attack (~> 4.4.1)