switch to unicode escape sequences for compatibility with strict mode

[awalgarg]

Presently, when requiring less for pragmatic usage with strict mode enabled, a syntax error is thrown because of use of octal escape sequences which are disabled in ES5 strict mode.

> node --use_strict
> var less = require('less');
/home/awal/dev/someProject/node_modules/less/lib/less-node/lessc-helper.js:19
               '\033[' + styles[style][1] + 'm';
                 ^^

This patch replaces them with the more universal unicode escape sequences, which fixes the above issue. I wasn't able to find any other places where strict-mode would throw at runtime, but I'd be happy to fix them as I find them and submit additional PRs if the feedback for this one is positive.

I realize this is not a priority and might not even be worth fixing for some, but our current setup enforces strict mode. It'd be great if this could be landed within less.js :)

[awalgarg]

Hmm, the appveyor failure for node 4 looks unrelated to this patch - or am I missing something?

[SomMeri]

Appveyor was a bit unstable lately, most likely, you are not missing anything.

[awalgarg]

Anything here that I need to improve? :)

[matthew-dean]

#2866 was opened with pretty much to same thing, so I merged that one.