Allow to pick old versions when running `self:update`, aka. roll-back

[gggeek]

Funnily enough, the self:update command seems to allow the user to override the current version, but it does not allow her to specify a specific version to upgrade (or downgrade) to.

It would be quite useful to be able to specify an old version as target, ie. rollback, when the latest version has f.e. introduced a bug which breaks the workflow of the developer. Atm I see no easy option for rolling back...

[pjcdawkins]

Agreed

Currently rollback can be done with:

mv ~/.platformsh/bin/platform-old.phar ~/.platformsh/bin/platform \
&& chmod +x ~/.platformsh/bin/platform \
&& platform -V