Issue title: deployment slot
 Issue body: hi,
When trying to use this with azure and then creating a deployment slot and copying production it does not work, just gets an internal error. any ideas how to set this up.
 Comments: 
 Comment 0: Hey, without the actual error, there's little I can do to help you. Could you go and check? You should find it in the iisnode folder of your Ghost deployment. Quick tutorial on how to get there:

1) YOURDOMAIN.scm.azurewebsites.net
2) Go to the debug console
3) `cd` into your Ghost folder
4) Find the `iisnode` folder with logs there
 Comment 1: thanks for getting back to me.


ERROR: casper cannot be activated because it is not currently installed. 
 
 Error
    at Error.ValidationError (D:\home\site\wwwroot\core\server\errors\validation-error.js:6:18)
    at then (D:\home\site\wwwroot\core\server\data\validation\index.js:120:35)
    at tryCatcher (D:\home\site\wwwroot\node_modules\bluebird\js\main\util.js:26:23)
    at Promise._settlePromiseFromHandler (D:\home\site\wwwroot\node_modules\bluebird\js\main\promise.js:507:31)
    at Promise._settlePromiseAt (D:\home\site\wwwroot\node_modules\bluebird\js\main\promise.js:581:18)
    at Promise._settlePromiseAtPostResolution (D:\home\site\wwwroot\node_modules\bluebird\js\main\promise.js:245:10)
    at Async._drainQueue (D:\home\site\wwwroot\node_modules\bluebird\js\main\async.js:128:12)
    at Async._drainQueues (D:\home\site\wwwroot\node_modules\bluebird\js\main\async.js:133:10)
    at Async.drainQueues (D:\home\site\wwwroot\node_modules\bluebird\js\main\async.js:15:14)
    at process._tickDomainCallback (node.js:463:13) 


this is the file i have done npm install so is weird that it cant find casper
 Comment 2: Could you try installing again? Clearly, something went wrong.
{'source': 'GithubIssues', 'repo_name_with_owner': 'felixrieseberg/Ghost-Azure', 'issue_id': 122157795, 'repo_stars': 153.0}