Bump dependencies to latest versions available; expand travis testing

Because of the bump in mocha version to 8.0, we can no longer run tests
on Node.js v8.x and below: mochajs/mocha#4164

The fibers package at version 5.0 also doesn't support Node.js v10.x
or lower, although it seems to work fine in my limited testing.

.travis.yml

@@ -1,9 +1,7 @@
 language: node_js
 sudo: false
 node_js:
+  - "unstable"
   - "stable"
-  - "8"
-  - "6"
-  - "4"
-  - "0.12"
-  - "0.10"
+  - "14"
+  - "12"

package.json

@@ -22,11 +22,11 @@
     "test": "test"
   },
   "dependencies": {
-    "commander": "^2.8.1"
+    "commander": "^6.0.0"
   },
   "devDependencies": {
-    "fibers": "~1.0.6",
-    "mocha": "~2.2.5"
+    "fibers": "^5.0.0",
+    "mocha": "^8.1.0"
   },
   "scripts": {
     "test": "mocha"