Improve caching for paketBootstrap

[Larusso]

Description

The paketBootstrap task was written two years ago with a complete different version of gradle in mind. Back then gradle did not invalidate the build output based on it's own build history. This is no real issue in everyday work with this plugin but makes considerable issues during test runs. Github rate limits the URL calls to the bootstrapper binary which results in a lot of test failures. These tests could work with a cached version of the paket.bootstrapper.exe and paket.exe. But in the current form each test run will redownload and setup both paket.bootstrapper.exe and paket.exe for every test spec example. This needs to be adressed as the offical linux support #50 is untestable at the moment.