Allow reinstall with make install (#1033)

Makefile

@@ -56,6 +56,8 @@ install-manpages: $(MANPAGES)
 	install -D -t $(MANDIR) $(MANPAGES)
 
 install: install-mnexec install-manpages
+#	This seems to work on all pip versions
+	$(PYTHON) -m pip uninstall mininet || true
 	$(PYTHON) -m pip install .
 
 develop: $(MNEXEC) $(MANPAGES)