swap sqlite3 & mysql dependencies (#7677)

no issue

- Ghost-CLI's recommended system stack has MySQL as the default
DB engine of choice, making the sqlite requirement unnecessary.
- Mysql (as the default) should be a required dependency

package.json

@@ -64,6 +64,7 @@
     "moment": "2.15.2",
     "moment-timezone": "0.5.9",
     "multer": "1.2.0",
+    "mysql": "2.11.1",
     "nconf": "0.8.4",
     "netjet": "1.1.3",
     "node-uuid": "1.4.7",
@@ -79,14 +80,13 @@
     "sanitize-html": "1.13.0",
     "semver": "5.3.0",
     "showdown-ghost": "0.3.6",
-    "sqlite3": "3.1.8",
     "superagent": "2.3.0",
     "unidecode": "0.1.8",
     "validator": "5.7.0",
     "xml": "1.0.1"
   },
   "optionalDependencies": {
-    "mysql": "2.11.1"
+    "sqlite3": "3.1.8"
   },
   "devDependencies": {
     "grunt": "1.0.1",