explicit protocol in configs, otherwise breaks in Electron

when it tries file://

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

Author: t3chguy

config.sample.json

@@ -12,7 +12,7 @@
         ]
     },
     "piwik": {
-        "url": "//piwik.riot.im/",
+        "url": "https://piwik.riot.im/",
         "siteId": 1
     }
 }

electron_app/riot.im/config.json

@@ -14,7 +14,7 @@
         ]
     },
     "piwik": {
-        "url": "//piwik.riot.im/",
+        "url": "https://piwik.riot.im/",
         "siteId": 1
     }
 }