Update to grape 1.4

[kubaracek]

Having troubles running this on grape 1.4.0 as the base response complains on

NoMethodError:
        undefined method `body' for #<Array:0x00000000046366a0>

It seems like the response changed and the second position of rack response now returns body directly (not a response object)

Tests are passing and works with my rails app using grape 1.4