can specify FooMigration as well as foo_migration

previously, if you did this:

    rake db:new_migration name=foo_migration

your migration's class name was FooMigration, but if you did this:

    rake db:new_migration name=FooMigration

your migration's class name was Foomigration. simple fix.

lib/tasks/standalone_migrations.rb

@@ -99,6 +99,6 @@ def file_name migration
   end
 
   def class_name str
-    str.split('_').map { |s| s.capitalize }.join
+    str.underscore.split('_').map { |s| s.capitalize }.join
   end
 end