add a soft puma dependency so people can run Discourse on puma

Gemfile

@@ -196,6 +196,7 @@ gem 'redis-rack-cache', git: 'https://github.com/SamSaffron/redis-rack-cache.git
 gem 'rack-cache', require: false
 gem 'rack-cors', require: false
 gem 'unicorn', require: false
+gem 'puma', require: false
 
 # perftools only works on 1.9 atm
 group :profile do

Gemfile.lock

@@ -287,6 +287,8 @@ GEM
       pry (~> 0.9.10)
     pry-rails (0.2.2)
       pry (>= 0.9.10)
+    puma (2.5.1)
+      rack (>= 1.1, < 2.0)
     qunit-rails (0.0.3)
       railties (>= 3.2.3)
     rack (1.4.5)
@@ -501,6 +503,7 @@ DEPENDENCIES
   pg
   pry-nav
   pry-rails
+  puma
   qunit-rails
   rack-cache
   rack-cors

Gemfile_rails4.lock

@@ -327,6 +327,8 @@ GEM
       pry (~> 0.9.10)
     pry-rails (0.3.1)
       pry (>= 0.9.10)
+    puma (2.5.1)
+      rack (>= 1.1, < 2.0)
     qunit-rails (0.0.3)
       railties (>= 3.2.3)
     rack (1.5.2)
@@ -511,6 +513,7 @@ DEPENDENCIES
   pg
   pry-nav
   pry-rails
+  puma
   qunit-rails
   rack-cache
   rack-cors