Use explicit Node version numbers in CI config

Author: xzyfer

.travis.yml

@@ -36,28 +36,28 @@ jobs:
       node_js: "9"
       os: osx
     - stage: platform-test
-      node_js: "7"
+      node_js: "8"
       os: linux
     - stage: platform-test
-      node_js: "7"
+      node_js: "8"
       os: osx
     - stage: platform-test
-      node_js: "lts/carbon"
+      node_js: "7"
       os: linux
     - stage: platform-test
-      node_js: "lts/carbon"
+      node_js: "7"
       os: osx
     - stage: platform-test
-      node_js: "lts/boron"
+      node_js: "6"
       os: linux
     - stage: platform-test
-      node_js: "lts/boron"
+      node_js: "6"
       os: osx
     - stage: platform-test
-      node_js: "lts/argon"
+      node_js: "4"
       os: linux
     - stage: platform-test
-      node_js: "lts/argon"
+      node_js: "4"
       os: osx
     - stage: platform-test
       node_js: "0.12"