Dockerfile: Update to current Pelias contact email address

Per https://github.com/pelias/pelias/tree/master/announcements/2018-01-02-pelias-update
replace @mapzen.com email address in Dockerfile with the current Pelias contact email address.

For example this will be visible from `docker inspect` with the other labels.

Dockerfile

@@ -2,7 +2,7 @@
 FROM pelias/baseimage
 
 # maintainer information
-LABEL maintainer="pelias@mapzen.com"
+LABEL maintainer="pelias.team@gmail.com"
 
 EXPOSE 3100