[[CHORE]] Improve CI configuration (#3415)

* [[CHORE]] Improve CI configuration

Use the "build matrix" feature offered by TravisCI in order to simplify
the "script" parameter and to create distinct jobs for the "test-262"
and "test-browser" commands (these were previously run as part of the
job scheduled to run the project's unit tests in Node.js version 10).

* debug

* Disable caching of npm modules in CI

> Please note that as of July 2019, npm is cached by default on Travis
> CI

https://docs.travis-ci.com/user/caching/#npm-cache

* Revert "debug"

This reverts commit ac4189f.

.travis.yml

@@ -9,16 +9,26 @@ before_install:
     fi
 script:
   - npm run pretest
-  - npm run test-node
-  - if [[ $(node --version) == v10* ]] ; then npm run test-262; fi
-  - if [[ $(node --version) == v10* ]] ; then npm run test-browser; fi
-node_js:
-  - "0.10"
-  - "0.12"
-  - 4
-  - 6
-  - 8
-  - 10
+  - npm run $CMD
+cache:
+  npm: false
 matrix:
+  include:
+    - node_js: "0.10"
+      env: CMD=test-node
+    - node_js: "0.12"
+      env: CMD=test-node
+    - node_js: 4
+      env: CMD=test-node
+    - node_js: 6
+      env: CMD=test-node
+    - node_js: 8
+      env: CMD=test-node
+    - node_js: 10
+      env: CMD=test-node
+    - node_js: 10
+      env: CMD=test-262
+    - node_js: 10
+      env: CMD=test-browser
   fast_finish: true
 sudo: false