drop node 6 support

We will no longer test on node6 and instead focus on 8+. This won't
break node6 immediately, but we will no longer go out of our way
supporting a version that does not receive security updates.

Signed-off-by: Claudius <opensource@amenthes.de>

package.json

@@ -119,7 +119,7 @@
     "socket.io": "~2.1.1",
     "socket.io-client": "~2.1.1",
     "spin.js": "^2.3.2",
-    "sqlite3": "^4.0.1",
+    "sqlite3": "^4.0.7",
     "store": "^2.0.12",
     "string": "^3.3.3",
     "tedious": "^1.14.0",
@@ -141,7 +141,7 @@
     "**/request": "^2.88.0"
   },
   "engines": {
-    "node": ">=6.x"
+    "node": ">=8.x"
   },
   "bugs": "https://github.com/codimd/server/issues",
   "keywords": [