return zero as timestamp when it does not present in riak cache(cherry picked from commit d4c290959524cb2bac2109f6867a0500ec8e7d9e)
minimize replica count for faster tile removal in riak cache(cherry picked from commit ecbdbd9d9a59ed71dd8626a8a4798e4fc6c763e2)
handle exceptions when storing/removing tiles in riak(cherry picked from commit c29dea83284763a13a7d8f652111300e6496cd3b)
fix syntax error, remove unused import, upgrade dependencies for travis ci
(cherry picked from commit f1504481a55da04c2206cf9c9645b1d9df8316b6)

Conflicts:

	.travis.yml