Use https instead of git for tether-drop

git protocol (tcp/udp/9418) is forbidden when using some corporate
proxies. HTTPS is used more widely.

package.json

@@ -78,7 +78,7 @@
     "sinon": "1.17.6",
     "systemjs-builder": "^0.15.34",
     "tether": "^1.4.0",
-    "tether-drop": "git://github.com/torkelo/drop",
+    "tether-drop": "https://github.com/torkelo/drop",
     "tslint": "^4.0.2",
     "typescript": "^2.1.4",
     "virtual-scroll": "^1.1.1"