strip initial `-` from the name of shell process

Avoids the case where shell detection would result in
`basename "-bash"` invocation, triggering the error:

    basename: illegal option -- b

fixes #462

libexec/rbenv-init

@@ -23,6 +23,7 @@ done
 shell="$1"
 if [ -z "$shell" ]; then
   shell="$(ps c -p $(ps -p $$ -o 'ppid=' 2>/dev/null) -o 'comm=' 2>/dev/null || true)"
+  shell="${shell##-}"
   shell="$(basename "${shell:-$SHELL}")"
 fi