[plt-scheme] Version number in add-ons

["Paul Steckler" <steck@ccs.neu.edu>]

For v200alpha19, I decided to make alphas available for MysterX,
SrPersist, and MzCOM.  Before releasing them, I neglected to update the
version numbers in their info.ss files (that task is now added to the
release checklist).  So, if you downloaded those packages, and run the
version checker from DrScheme, you'll get a warning that those packages
are out-of-date.

Easy fix: go into collects/{mysterx,srpersist,mzcom}/info.ss and change
the version number to 200alpha19.

-- Paul