Update for tsinfer version 0.4

[jeromekelleher]

Tests are currently broken for tsinfer version 0.4.0a1:

self = <sc2ts.inference.Matcher object at 0x7fb41c9a0430>, sample_indexes = array([0])         
                                                                                              
    def _match_samples(self, sample_indexes):                                                 
        # Some hacks here to work around the fact that tsinfer does a bunch                    
        # of stuff we don't want here. All we want are the matched paths and                   
        # mutations.                           
        num_samples = len(sample_indexes)                                                     
        self.match_progress = self.progress_monitor.get("ms_match", num_samples)              
        if self.num_threads <= 0:                                                             
>           self._SampleMatcher__match_samples_single_threaded(sample_indexes)                
E           AttributeError: 'Matcher' object has no attribute '_SampleMatcher__match_samples_single_threaded'

We can pin to version == 0.3 for now, and update later when 0.4.0 is released.