update coverage html report to coverage 4.0

private `_harvest_data()` is now `get_data()`

IPython/testing/iptestcontroller.py

@@ -453,7 +453,7 b' def run_iptestall(options):'
                         cu.name = '.'.join(nameparts[ix:])
 
             # Reimplement the html_report method with our custom reporter
-            cov._harvest_data()
+            cov.get_data()
             cov.config.from_args(omit='*{0}tests{0}*'.format(os.sep), html_dir=html_dir,
                                  html_title='IPython test coverage',
                                 )