Check for updates when starting a new shell.

This reinstates the original behaviour of checking for updates (which is included in `parts init`) when starting a new shell.
Because `parts_init` var is changed here, line 32 `file.include? parts_init` condition will mean that the file can potentially have two lines, one for evalling `parts env` the other for evalling `parts init`.

Which I guess is fine, but something to watch out for in the future if we change it back.