Depend on gjs (>= 1.50.2-3~)

So that using an object after it's destroyed doesn't crash the
Shell. Critical warnings will be logged instead, usually to the systemd
journal. Please report these as new bugs if seen: we can't tell from the
crash backtraces whether the bug was in the Shell itself or an extension,
which makes the existing bug reports unclear, but the critical warnings
should make it clearer where the bug is. (Closes: #881301, #880663; LP:
#1714989, LP: #1721321, LP: #1722986, LP: #1725015)