how do I get the dependencies right between a branch of stan and lib/stan_math?

this PR is failing: https://github.com/stan-dev/stan/pull/2244

I started this branch in order to add vector and matrix expressions - this required adding a specialization to lib/stan_math that was merged into the develop branch.

the PR is testing against the old version of lib/stan_math, which fails.

what is the proper incantation to make it run against the current develop branch of lib/stan_math?

is this in the wiki? couldnâ€™t find it.