Update TravisCI config

Adds Node 4.x versions, and containerized builds (faster).

Connects pelias/pelias#258

.travis.yml

@@ -1,6 +1,16 @@
+sudo: false
 language: node_js
-script: "npm run unit"
 node_js:
-  - "0.10"
-  - "0.12"
-sudo: false
+  - 0.10
+  - 0.12
+  - 4.0
+  - 4.1
+  - 4.2
+matrix:
+    allow_failures:
+        - node_js: 4.0
+        - node_js: 4.1
+        - node_js: 4.2
+env:
+    - TEST_SUITE=unit
+script: "npm run $TEST_SUITE"