Since we don't want to build packages for pull requests (as they cannot
get archived as artifacts), we need a slightly different build command,
and moving it to a script was cleaner.
Linux and OS X no longer install packages, instead they download the
1.0.0 uninstall and install scripts provided by .NET CLI, uninstall all
existing `dotnet` packages, and "install" the latest 1.0.0 preview2
version of `dotnet` to `~/.dotnet`, which `Start-PSBuild` automatically
picks up.
Additionally, `Start-PSBootstrap` only attempts to install dependencies
when it detects that it is on Ubuntu 14.04, otherwise it warns that
dependencies must be installed manually, and still installs `dotnet` via
their obtain script. This makes `Start-PSBootstrap` safe to run on
Debian, and usable on CentOS.
