Bump puma from 3.9.1 to 3.12.6

Bumps [puma](https://github.com/puma/puma) from 3.9.1 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@v3.9.1...v3.12.6)

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

Gemfile

@@ -7,7 +7,7 @@ git_source(:github) do |repo_name|
 end
 
 gem 'rails',        '5.1.4'
-gem 'puma',         '3.9.1'
+gem 'puma',         '3.12.6'
 gem 'sass-rails',   '5.0.6'
 gem 'uglifier',     '3.2.0'
 gem 'coffee-rails', '4.2.2'

Gemfile.lock

@@ -111,7 +111,7 @@ GEM
     pry (0.11.3)
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
-    puma (3.9.1)
+    puma (3.12.6)
     rack (2.0.5)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
@@ -204,7 +204,7 @@ DEPENDENCIES
   listen (= 3.0.8)
   minitest-reporters (= 1.1.14)
   pg (= 0.18.4)
-  puma (= 3.9.1)
+  puma (= 3.12.6)
   rails (= 5.1.4)
   rails-controller-testing (= 1.0.2)
   sass-rails (= 5.0.6)