Enforce supported versions of node and npm

This package uses node features which require node v6.x and higher.
Prior to this change the package would install but a user would
get runtime errors for constructs (e.g. class) which are not
natively supported in prior version of node.  For compatibility with
prior versions of npm < 3.0.0, included both the "engine-strict" and
"engineStrict" (deprecated) flags.

Fixes FAB-725

Change-Id: Ib7042ecdec51bdcfddbd6be23f69d5391519572d
Signed-off-by: Gari Singh <[email protected]>

package.json

@@ -9,6 +9,9 @@
   "scripts": {
     "test": "node test/unit/headless-tests.js"
   },
+  "engines": {"node": ">=6.7.0","npm":">=3.0.0"},
+  "engine-strict": true,
+  "engineStrict": true,
   "dependencies": {
     "aes-js": "^1.0.0",
     "asn1": "https://github.com/mcavage/node-asn1",