deb/Makefile: added 'clean'.

deb/Makefile

@@ -1,8 +1,11 @@
 COMMIT?=HEAD
 
-.PHONY: package
+.PHONY: package clean
 
 package:
 	vagrant up
 	vagrant ssh -c 'rm -rf pandoc && git clone https://github.com/jgm/pandoc && cd pandoc && git checkout -b work $(COMMIT) && git submodule update --init && ./make_deb.sh && cp *.deb /vagrant_data/'
 	vagrant halt
+
+clean:
+	vagrant destroy