Update to the lastest rickshaw

Since
https://github.com/shutterstock/rickshaw/commit/417d44847eb750c8e11cc375f39d72a53dc7ba71
min=auto is working properly even if data contains null values.
Fix several zero vs null issues.

This fixes:
- "last" summary when values are zero;
- genuine zero values disappearing if null_as=null.
Merge pull request #13 from knyar/zero-vs-null

Fix several zero vs null issues
Merge pull request #12 from knyar/master

Update to latest rickshaw