Fix running on Node v0.10.48

[Jimbly]

Attempting to require this module on Node v0.10 gives the following error:

yazl/index.js:391
    this.setFileAttributesMode(isDirectory ? 0o40775 : 0o100664);
                                             ^
SyntaxError: Unexpected token ILLEGAL

Octal literals were not added until ES2015 and not available in Node v0.10.x.

If you run a test through istanbul cover, this error apparently does not occur because the instrumentation for coverage reporting is massaging that constant into decimal:

this.setFileAttributesMode(isDirectory ? (__cov_3jDDzS4sgHbw9aLrC2MDAg.b['61'][0]++, 16893) : (__cov_3jDDzS4sgHbw9aLrC2MDAg.b['61'][1]++, 33204));

It directly and immediately fails when trying to run node test/test.js or require('yazl') in Node v0.10.48.

This appears to have been broken for years and no one complained though, so may I instead propose removing v0.10.48 from .travis.yml? =)

Using the old 040775 format directly probably isn't a good idea, since that's a syntax error in strict mode, which modules are loaded as by default in newer versions of Node if using the experimental ES modules features, I believe.

Resolves #61