add depends on octave 4.4 for classdef

[mtmiller]

I tried installing testify in older versions of Octave, and wound up with errors, either at install time or at run time when calling functions. So to make things easier, add a Depends field to stop users from trying to use it with older versions.

As an example, in Octave 4.2

>> test2 version
parse error near line 861 of file /home/mike/.local/share/octave/4.2/packages/testify-0.3.3/+testify/+internal/BistRunner.m

  syntax error

>>> function out = trimleft (str)
           ^

error: called from
    test2 at line 162 column 8

With this PR, in Octave 4.2

>> pkg install testify.tar.gz
error: the following dependencies were unsatisfied:
   testify needs octave >= 4.4

[apjanke]

Good idea. Merged.

Thanks!