add gem pg

Gemfile

@@ -59,7 +59,7 @@ group :test do
 end
 
 group :production do
-  gem 'mysql2', '0.5.3'
+ gem 'pg','1.2.3'
 end
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
 gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

Gemfile.lock

@@ -106,6 +106,7 @@ GEM
     nio4r (2.5.2)
     nokogiri (1.10.10)
       mini_portile2 (~> 2.4.0)
+    pg (1.2.3)
     pry (0.13.1)
       coderay (~> 1.1)
       method_source (~> 1.0)
@@ -208,6 +209,7 @@ DEPENDENCIES
   listen (>= 3.0.5, < 3.2)
   mysql2 (= 0.5.3)
   nokogiri
+  pg (= 1.2.3)
   pry
   puma (~> 3.11)
   rails (~> 5.2.4, >= 5.2.4.3)