Do not display "ENSIME" in modeline when connected

"ENSIME" always took some space in the Emacs mode line, although the doc
string of `ensime-modeline-string` says that "ENSIME" only appears if we
are not connected. This often caused the more meaningful state
information to be pushed off the screen.
Merge pull request #324 from dkim/ensime-modeline-string

Do not display "ENSIME" in modeline when connected