ci: build with current Node LTS & master

Also drops support for unmaintained Node versions.

Credit: @helio-frota
Credit: @elidoran
Reviewed-By: @othiym23
PR-URL: #71
PR-URL: #78

Author: helio-frota

.travis.yml

@@ -1,9 +1,8 @@
 language: node_js
-language: node_js
 node_js:
-  - '0.8'
-  - '0.10'
   - '0.12'
-  - 'iojs'
+  - '4'
+  - '6'
+  - '7'
 before_install:
   - npm install -g npm@latest