[1.7.x] Added dates to release notes.

Django 1.7.8 release notes

==========================

*Under development*

*May 1, 2015*

Django 1.7.8 fixes:

* A database table name quoting regression in 1.7.2 (:ticket:`24605`).

* Prevented the loss of ``null``/``not null`` column properties during field

  alteration of MySQL databases (:ticket:`24595`).

* The loss of ``null``/``not null`` column properties during field alteration

  of MySQL databases (:ticket:`24595`).