Use (nrepl-current-ns) rather than nrepl-buffer-ns for interactive eval

When visiting a file for the first time, nrepl-buffer-ns is set to
"user" which made interactive eval commands such as
nrepl-eval-last-expression slightly surprising as they wouldn't be
evaluated in the current buffer's namespace. This patch now
uses (nrepl-current-ns) instead which should do the right thing.