Fixing Placeholder Script's Handle

The jQuery script we use for placeholders is actually [Simple Placeholder](https://github.com/marcgg/Simple-Placeholder) which is far less common (45 stars compared to 1,957 stars) than [jQuery Placeholder](https://github.com/mathiasbynens/jquery-placeholder). So when other plugins enqueue the jQuery Placeholder script there are some conflicts.

To mitigate this I just changed the handle from `jquery-placeholder` to `jquery-simple-placeholder`. Now if other plugins or themes use the mathiasbynens' version of the placeholder script there shouldn't be any problems.

See here: http://wordpress.org/support/topic/jquery-error-has-not-method-simpleplaceholder?replies=1
And here: https://woothemes.zendesk.com/agent/#/tickets/83703
for reference.