Bump puma from 3.8.0 to 3.12.6

Bumps [puma](https://github.com/puma/puma) from 3.8.0 to 3.12.6.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.8.0...v3.12.6)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -60,5 +60,5 @@ end
 group :production do
   gem 'pg', '~> 0.15'
   gem 'rails_12factor'
-  gem 'puma',           '3.8'
+  gem 'puma',           '3.12.6'
 end

Gemfile.lock

@@ -98,7 +98,7 @@ GEM
       racc (~> 1.4)
     orm_adapter (0.5.0)
     pg (0.21.0)
-    puma (3.8.0)
+    puma (3.12.6)
     racc (1.5.2)
     rack (1.6.13)
     rack-test (0.6.3)
@@ -200,7 +200,7 @@ DEPENDENCIES
   jbuilder (~> 2.0)
   jquery-rails
   pg (~> 0.15)
-  puma (= 3.8)
+  puma (= 3.12.6)
   rails (= 4.2.8)
   rails_12factor
   rake (~> 13.0.1)