tests: test against more relevant versions of node on Travis (#251)

* Test against more relevant versions of node

- allow for `node@unstable` failures
- slightly quicker npm install

* Remove "unstable" from Node versions

Waiting on nvm-sh/nvm#1053 for nightly installs.

* Keep node@0.10

* Test against node@5 and node@0.12

.travis.yml

@@ -1,11 +1,17 @@
 sudo: false
 language: node_js
 node_js:
-  - '0.12'
-  - '0.10'
-  - iojs
+  - 4
+  - 5
+  - "0.10"
+  - "0.12"
+  - "stable"
+
+before_install:
+  - "npm config set progress=false"
+
 env:
-- secure: "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"
+  secure: "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"
 
 after_script:
-  - 'cat ./coverage/lcov.info | ./node_modules/.bin/coveralls'
+  - "cat ./coverage/lcov.info | ./node_modules/.bin/coveralls"