Register supported adapters, fixes loading gem from local path

without this for some reason, rails thinks it needs CRuby gems such
as pg gem, etc.

Author: JesseChavez

lib/arjdbc.rb

@@ -12,8 +12,20 @@
   rescue LoadError => e
     warn "activerecord-jdbc-adapter failed to load railtie: #{e.inspect}"
   end if defined?(Rails) && ActiveRecord::VERSION::MAJOR >= 3
+
+  ActiveSupport.on_load(:active_record) do
+    ActiveRecord::ConnectionAdapters.register(
+      "sqlite3", "ActiveRecord::ConnectionAdapters::SQLite3Adapter", "arjdbc/sqlite3/adapter"
+    )
+    ActiveRecord::ConnectionAdapters.register(
+      "postgresql", "ActiveRecord::ConnectionAdapters::PostgreSQLAdapter", "arjdbc/postgresql/adapter"
+    )
+    ActiveRecord::ConnectionAdapters.register(
+      "mysql2", "ActiveRecord::ConnectionAdapters::Mysql2Adapter", "arjdbc/mysql/adapter"
+    )
+  end
 else
   warn "activerecord-jdbc-adapter is for use with JRuby only"
 end
 
-require 'arjdbc/version'
+require 'arjdbc/version'