update grape to 0.3.1 for ruby 2.0 support.

grape gem < 0.3.1 does not work on ruby 2.0 because of #respond_to? behaviour.

I confirmed that all spinach and spec tests passed on ruby 1.9.3p392 with this change, and also confirmed that all spec tests related to API passed on ruby 2.0.0p0.

Still 1 spinach test failure and 3 spec tests failures remain on ruby 2.0.0p0, but it should be safe to merge this change first.