Update Travis for handle N-API builds

Since N-API is a versioned API the binding code is specific to
a particular version. In this case it's current N-API v3 which
only ships in Node 10 at the moment.

This means the binary only builds on Node 10, but runs on N-API
enabled (and shimmed) Node versions. As a result we need to build
the binaries upfront on Node 10 and share it across all test runs.

Author: xzyfer

.travis.yml

@@ -3,38 +3,39 @@ language: node_js
 compiler: gcc
 sudo: false
 
-env:
-  global:
-    - SKIP_SASS_BINARY_DOWNLOAD_FOR_CI=true
+stages:
+  - lint
+  - build-binary
+  - test
 
 jobs:
   include:
-    - stage: test
-      node_js: "10"
+    - stage: lint
       os: linux
-      before_script: npm run lint || exit 1;
-      after_success: npm run-script coverage;
-    - stage: platform-test
-      node_js: "lts/boron"
+      script: npm run lint || exit 1;
+
+    - stage: build-binary
       os: linux
-    - stage: platform-test
+      script: npm run-script build -f
+    - os: osx
+      script: npm run-script build -f
+
+    - stage: test
       node_js: "lts/boron"
-      os: osx
-    - stage: platform-test
-      node_js: "lts/carbon"
       os: linux
-    - stage: platform-test
-      node_js: "lts/carbon"
+    - node_js: "lts/boron"
       os: osx
-    - stage: platform-test
-      node_js: "9"
+    - node_js: "lts/carbon"
       os: linux
-    - stage: platform-test
-      node_js: "9"
+    - node_js: "lts/carbon"
       os: osx
-    - stage: platform-test
-      node_js: "10"
+    - node_js: "9"
+      os: linux
+    - node_js: "9"
       os: osx
+    - os: linux
+    - os: osx
+      after_success: npm run-script coverage;
 
 addons:
   apt:
@@ -79,6 +80,7 @@ cache:
     - $HOME/.node-gyp
     - $HOME/.npm
     - node_modules
+    - vendor
 
 notifications:
   webhooks: