Update version number

- Update version number to 2.0.0 since changes affect backwards
  compatibility with node v4.
- Update Travis config to allow node module caching.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "base58",
-  "version": "1.0.1",
+  "version": "2.0.0",
   "keywords": [
     "base58, flickr"
   ],
@@ -35,6 +35,6 @@
   },
   "scripts": {
     "lint": "eslint . --fix",
-    "test": "mocha test"
+    "test": "mocha"
   }
 }