Upgrade curator to 4.2.6

Dockerfile

@@ -1,7 +1,7 @@
 FROM alpine:3.4
 
 RUN apk --update add python py-setuptools py-pip && \
-    pip install elasticsearch-curator==4.2.4 && \
+    pip install elasticsearch-curator==4.2.6 && \
     apk del py-pip && \
     rm -rf /var/cache/apk/*