Remove unused mysql2 dependency

Gemfile

@@ -3,7 +3,6 @@
 source 'https://rubygems.org'
 
 gem 'pg', '~> 1.6'
-gem 'mysql2', '~> 0.5'
 
 gem 'rubyzip', '~> 3.2'
 

Gemfile.lock

@@ -1,20 +1,24 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    bigdecimal (3.3.1)
+    bigdecimal (4.0.1)
+    io-console (0.8.2)
     mize (0.6.1)
-    mysql2 (0.5.7)
-      bigdecimal
-    optparse (0.8.0)
+    optparse (0.8.1)
     pg (1.6.2)
     pg (1.6.2-x86_64-linux)
+    readline (0.0.4)
+      reline
+    reline (0.6.3)
+      io-console (~> 0.5)
     rubyzip (3.2.2)
     sync (0.5.0)
     term-ansicolor (1.11.3)
       tins (~> 1)
-    tins (1.47.0)
+    tins (1.51.0)
       bigdecimal
       mize (~> 0.6)
+      readline
       sync
 
 PLATFORMS
@@ -22,7 +26,6 @@ PLATFORMS
   x86_64-linux
 
 DEPENDENCIES
-  mysql2 (~> 0.5)
   optparse (~> 0.8.0)
   pg (~> 1.6)
   rubyzip (~> 3.2)