🐛 fix brute schema

no issue

- brute-knex uses timestamp type, see https://github.com/llambda/brute-knex/blob/master/index.js
- so we also defined timestamp type
- see http://stackoverflow.com/questions/9192027/invalid-default-value-for-create-date-timestamp-field
- see http://stackoverflow.com/questions/35237278/mysql-invalid-default-value-for-timestamp-when-no-default-value-is-given
- mysql does not allow a second timestamp without default value
- we have to options: define a default value or allow null or use dateTime
- let's use dateTime, as 1. we are using it for all our dates in Ghost and 2. it's recommended
- read here http://www.sqlteam.com/article/timestamps-vs-datetime-data-types
- there are some difference, i think the most important difference is that TIMESTAMP changes if the tz changes in your database
- lifetime is timestamp not a bigInteger, this was a mistake i think (see https://github.com/llambda/brute-knex/blob/master/index.js#L115)

core/server/data/schema/schema.js

@@ -236,9 +236,9 @@ module.exports = {
     },
     brute: {
         key: {type: 'string'},
-        firstRequest: {type: 'timestamp'},
-        lastRequest: {type: 'timestamp'},
-        lifetime: {type: 'bigInteger'},
+        firstRequest: {type: 'dateTime'},
+        lastRequest: {type: 'dateTime'},
+        lifetime: {type: 'dateTime'},
         count: {type: 'integer'}
     }
 };