Refs https://github.com/janl/mustache.js/pull/701

Also tried with Node.js 10 and 12, but some CLI tests explodes so awaiting
a fix for that before adding those versions explicitly.

Primarily because it's very painful to install 3rd party packages
in the test phase of our CI runs, because many of them requires at least
Node.js 0.10 or even higher.

Therefore getting the test suite to run when using Node.js 0.8 is
far from trivial, and the benefit of ensure Node.js 0.8 support these
days is not obvious because it should really not be used knowing that
it has reached end-of-life a long time ago.

Worth noting there's no breaking changes in this commit to
mustache.js's source code, we just don't want to struggle getting our
CI to care about it. In other words, it should still be possible to use
Node.js 0.8, but we won't care much making sure it keeps working going
forward.

For a long time we've struggled getting the browser test suite to run
on sauce labs. Those tests were running on Node.js 4 which is getting
quite old.

In a trial-n-error effort, I tried adding Node.js 6 as one of the
versions to run on Travis CI, and at the same time ensure the browsers
tests were run on that version instead of Node.js 4 -- voila the suite
passed right away.

It therefore seemed like the issues we were seeing had to do with us
running those tests via Node.js 4.

For a long time, we've been installing npm v2.x before running tests
on Travis CI across all Node.js versions.

While doing some testing today, I observed that we were able to install
dependencies and run all tests just fine on all Node.js versions, except
for v0.8. Therefore ensure we only force an install of npm v2.x when
running tests on Node.js 0.8, and keep the installed npm version as is
for all other Node.js versions.

As zuul consequently fails when running on node v0.12.

Use latest v2 release of npm as the just released v3 doesnt seem to be node v0.8 compliant atm.

And test on node v4 for obvious reasons.

As secure $ENVs are disabled for pull requests and those $ENVs are required for running browser tests on saucelabs.

http://docs.travis-ci.com/user/pull-requests/#Security-Restrictions-when-testing-Pull-Requests

+ updated readme with the fact that browser tests are only runned automatically when *merged into master*, not in the PR stage

With 25 different browser versions it could easily take 15-25 minutes for a run too finish.
Only test latest chrome & firefox atm, increase support later as needed.

Removing hacks to get v0.6 tests running; specific npm client version + upgrading mocha.
Also v0.12 of node has been released, therefore there's no reason to still keep testing on v0.11.

Covers the project with browser tests, not only V8 via node/iojs. Tests are runned with [zuul](https://github.com/defunctzombie/zuul) which
provides simple local browser testing, aswell as cloud testing on multiple browsers with [saucelabs](http://saucelabs.com).

- Documented required version of npm while developing
- Use the latest npm client for node >= v0.8 on travis

Special quirks to get node v0.6 supported:
	- npm client v1.3.26 on travis
	- jshint 2.4.4 as >= 2.5.x breaks

package name is now "xulrunner-2.0". We need this to run our tests
in SpiderMonkey.