remove default value from text column

When I clone the repo and run `bundle exec rake db:migrate`, I got the errors below:

```
Mysql2::Error: BLOB/TEXT column 'content' can't have a default value: CREATE TABLE `blogs` (`id` int(11) DEFAULT NULL auto_increment PRIMARY KEY, `title` varchar(30) DEFAULT '我的日志' NOT NULL, `content` text DEFAULT '' NOT NULL, `user_id` int(11) DEFAULT 0 NOT NULL, `tag` varchar(255), `created_at` datetime NOT NULL, `updated_at` datetime NOT NULL) ENGINE=InnoDB/Users/zhangyuan/code/My_blog/db/migrate/20131009153248_create_blogs.rb:4:in `up'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)
```

MySQL Server version: 5.6.16 Homebrew