Updated install instructions

This is the same as my previous pull, but for 5.0.
Because `~5` means get ANY version provided that it's at least 5.0.0. That means we're saying, yeh, go ahead and get guzzle 7 for example. This is not desirable because it will encourage people to use such a version constraint, and then will cause broken packages in the future due to people depending on guzzle `~5`. We shouldn't encourage it at any point.