(chore) Update dependencies, remove node.js 8 support

Author: Gyula Szalai

.travis.yml

@@ -1,7 +1,7 @@
 language: node_js
 node_js:
   - 'node'
-  - '6'
+  - '12'
 
 script: npm test
 

dist/src/JwtHandler.d.ts

@@ -7,7 +7,7 @@ export declare type PubkeyData = {
 export declare type PrivkeyData = {
     key: string;
     passphrase: string;
-    alg: string;
+    alg: jwt.Algorithm;
 } | undefined | null;
 export declare type PubkeyResolver = (keyId: string) => PubkeyData | Promise<PubkeyData>;
 export declare type PrivkeyResolver = (keyId: string) => PrivkeyData | Promise<PrivkeyData>;