Fix posix-spawn to version 0.3.12 (#7517)

Because 0.3.13 causes build error in x86 or other ILP32 environments.
closes #7453

Gemfile

@@ -61,7 +61,7 @@ gem 'nsa', '~> 0.2'
 gem 'oj', '~> 3.5'
 gem 'ostatus2', '~> 2.0'
 gem 'ox', '~> 2.9'
-gem 'posix-spawn', '~> 0.3'
+gem 'posix-spawn', '0.3.12'
 gem 'pundit', '~> 1.1'
 gem 'premailer-rails'
 gem 'rack-attack', '~> 5.2'

Gemfile.lock

@@ -387,7 +387,7 @@ GEM
     pghero (2.1.0)
       activerecord
     pkg-config (1.3.0)
-    posix-spawn (0.3.13)
+    posix-spawn (0.3.12)
     powerpack (0.1.1)
     premailer (1.11.1)
       addressable
@@ -711,7 +711,7 @@ DEPENDENCIES
   pg (~> 1.0)
   pghero (~> 2.1)
   pkg-config (~> 1.3)
-  posix-spawn (~> 0.3)
+  posix-spawn (= 0.3.12)
   premailer-rails
   private_address_check (~> 0.4.1)
   pry-byebug (~> 3.6)