Upgrade xargo if old

[jackcmay]

Problem

Install scripts only install xargo if xargo not already installed. If already installed the existing version will be used. The result is a mismatch of versions across installs that may not be compatible with the current SDK

Summary of Changes

Install xargo is not already installed or upgrade to latest if is

Fixes #

[CriesofCarrots]

@jackcmay , is the nightly change necessary?
(I ask because it looks like it's breaking solana-web3.js CI: https://travis-ci.org/solana-labs/solana-web3.js?utm_medium=notificatioxn&utm_source=github_status)

[jackcmay]

Bummer, the upgrade option is only available on nightly. The alternatives are either require nightly or do the "upgrade" manually (try install, check the version, force install if not latest). Not sure how to know the latest and don't want to force install every time a build is started...