Fix yarn compatibility

[ssbarnea]

As vsce version does not get updated magically, we need to upgrade
the minimum version in order to allow building projects with yarn.

Fixes #451
Related: microsoft/vscode-vsce#742

[ssbarnea]

@odockal What is weird is I seen some code inside packageExtension where it checks if vsce is installed on that parent level and is trying to use it.

The funny bit is that this logic did not work with a yarn based extension and it still attempted to run vsce from within vscode-extension-tester itself, which was older than the versions we wanted to use. Basically that feature did not work in that case.

        let vscePath = path.join(__dirname, '..', '..', 'node_modules', '.bin', 'vsce');
        if (!fs.existsSync(vscePath)) {
            vscePath = path.join('node_modules', '.bin', 'vsce');
        }

That is the bit of code that made it still use the outdated version from vscode-ansible/node_modules/vscode-extension-tester/node_modules/.bin/vsce instead of vscode-ansible/node_modules/.bin/vsce

If I understood the code correctly the order is exactly the opposite, as the version from within extest itself is preferred, so user does not have much control on how to use it.