Move translation file from es_AR to es

We can start differentiating among different flavours of Spanish once we
have more than one translation.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#	modified:   debian/changelog
#	modified:   po/Makefile
#	renamed:    po/es_AR.po -> po/es.po
#
