append colon (:) to end of brew's part of INFOPATH in brew shellenv

The behavior of some programs (e.g., emacs) differs depending on
whether INFOPATH is terminated in a colon or not. For example, the
info viewer built into emacs has a default list of places from which
it will draw info files, and it will only check this if either (a)
INFOPATH is unset, or (b) INFOPATH ends in a colon.

Currently brew shellenv prepends to the existing $INFOPATH and will
separate with a colon if $INFOPATH already exists, but if it does not
it will not terminate in a colon. As a result, info
pages (like emacs and elisp) are missing from the info viewer.

The existing brew shellenv logic used a parameter expansion trick to
include a colon only if it was prepending to an extant INFOPATH, but
because we want brew's contribution to the INFOPATH to always end in a
colon (whether there's an existing INFOPATH or not), we don't have to
use the parameter expansion trick

This commit only changes behavior for "other" shells (i.e., not fish,
csh, or tcsh) since I don't know how to append a colon to variables in
these shells.