Python 3 update for dropbox-cli

origin commit from:
dropbox/nautilus-dropbox#68
dropbox/nautilus-dropbox@a07a129

Author: Austin Kim

Makefile.am

@@ -6,11 +6,11 @@ EXTRA_DIST = caja-dropbox.in serializeimages.py caja-dropbox.txt.in docgen.py rs
 man_MANS = caja-dropbox.1
 
 caja-dropbox: caja-dropbox.in serializeimages.py
-	python serializeimages.py $(PACKAGE_VERSION) $(datadir)/applications < caja-dropbox.in > caja-dropbox
+	python3 serializeimages.py $(PACKAGE_VERSION) $(datadir)/applications < caja-dropbox.in > caja-dropbox
 	chmod +x caja-dropbox
 
 caja-dropbox.1: caja-dropbox.txt.in caja-dropbox docgen.py
-	python docgen.py $(PACKAGE_VERSION) caja-dropbox.txt.in caja-dropbox.txt
+	python3 docgen.py $(PACKAGE_VERSION) caja-dropbox.txt.in caja-dropbox.txt
 	$(RST2MAN) caja-dropbox.txt > caja-dropbox.1
 
 SUBDIRS = data src