Change minor mode init-value to nil.

According to the [Emacs documentation](http://www.gnu.org/software/emacs/manual/html_node/elisp/Defining-Minor-Modes.html), the `:init-value` for the minor mode should almost always be `nil`.

In this case, not setting it to `nil` is causing the ruby-block minor mode to be enabled in all buffers once ruby-block is enabled.