Merge pull request #141 from pgilad/master

Improve travis.yml and run on more subsets of node.js versions

.travis.yml

@@ -1,12 +1,14 @@
-
-{
-  "language": "node_js",
-  "node_js": "8",
-  "services": [
-    "mongodb"
-  ],
-  "script": [
-    "npm run build",
-    "npm run test"
-  ]
-}
+sudo: false
+language: node_js
+node_js:
+    - 8
+    - 9
+    - 10
+services:
+    - mongodb
+cache:
+    directories:
+        - node_modules
+script:
+    - npm run build
+    - npm run test