Upgrade grainstore dependency

Require the grainstore version allowing per-layer srid

This is needed to make previous commit effective

Version 2.7.0

2017-02-20

Announcements:
 - Upgrades cartodb-psql to [0.7.1](https://github.com/CartoDB/node-cartodb-psql/releases/tag/0.7.1).

Require the grainstore version allowing per-layer srid

This is needed to make previous commit effective

Version 2.6.5

2016-12-13

Announcements:
 - Upgrades request dependency.

Version 2.6.4

2016-12-13

Announcements:
 - Upgrades grainstore to [1.4.0](https://github.com/CartoDB/grainstore/releases/tag/1.4.0).

Version 2.6.3

2016-12-01

Announcements:
 - Upgrades grainstore to [1.3.0](https://github.com/CartoDB/grainstore/releases/tag/1.3.0).

Version 2.6.2

2016-11-05

Bug fixes:
 - Now validates all mapnik layers and if it fails then checks layer by layer to indicate which one fails.

Version 2.6.1

2016-11-01

Bug fixes:
 - Fix bad behavior in map validator, now validates layer by layer to indicate which one fails.

Version 2.6.0

2016-10-31

Enhancements:
 - Expose layer index if map validation fails.