Fixes migrations for MySQL5.

This removes the default value on the summary text column. Excerpt from
MySQL 5.0 Reference Manual, 11.1.6.3. The BLOB and TEXT Types: BLOB and
TEXT columns cannot have DEFAULT values [1].

This seems to cause problems on windows boxes since 2008 [2].

[1] http://dev.mysql.com/doc/refman/5.0/en/blob.html
[2] http://stackoverflow.com/a/4553664/1474757
Merge pull request #43 from opf/fix/mysql5

Fixes migrations for MySQL5.