Fix path.posix not being available in browsers

[t-mullen]

path-browserify does not provide a .posix implementation.

Environments without a posix implementation should just use the native implementation.

[ghost]

hyperdrive works in the browser when this patch is applied

[ralphtheninja]

@andrewosh Any reason for not getting this merged?

[ralphtheninja]

I just had a look at path-browserify and now it has a .posix property added, see https://github.com/browserify/path-browserify/blob/master/index.js#L527

So this PR can be closed. Solution is to update your browserify version to at least version 17.