remove DEFAULT-value on TEXT-type column.

When installing `schema.sql` into mysql-5.6.10, I got error (no error on mysql-5.5.24):

```
% ~/local/mysql56/bin/mysql -V
~/local/mysql56/bin/mysql  Ver 14.14 Distrib 5.6.10, for Linux (x86_64) using  EditLine wrapper
% cat schema.sql | ~/local/mysql56/bin/mysql -u root -p hrforecast
Enter password:
ERROR 1101 (42000) at line 2: BLOB/TEXT column 'meta' can't have a default value
```

so, removed DEFAULT-value on TEXT-type columns.

I guess this doesn't affect other versions. (passed on mysql-5.5.24)