Updated Casper to latest version

Fixes #1846
Remove successful login connections from the auth
throttle list

- once a user has successfully logged into ghost
they no longer are a malicious user and as such
their IP address should be removed from the
array of login attempts

- should also reduce the memory usage of Ghost
as the loginSecurity array gets pruned upon
every successful login

- this also fixes a race condition i was experiencing
during functional tests wherein i would receive
the login throttle message during regular testing.
Seems my machine is able to run casper fast enough
that it could complete each test under an amount
of time that tripped the login throttle message.
Merge pull request #1848 from schneidmaster/fix-1846

Updated Casper to latest version
Merge pull request #1850 from hswolff/fix-throttle-behavior

Remove successful login connections from the auth throttle list
New Post UX behaviour.

refs #1351
- Make generateSlug a static method on base model
- Change UI behavior to update placeholder
when a post has not yet been saved to the backend
- Add new getSlug API method
- prevent saving a new post’s slug/date/static status
before the post exists in the backend