Once libmongoc 1.5.0-rc2 is released, we should change the config on our CI systems to build against it on the 3.1-dev branch. This will stabilize our builds, which are currently plagued by the occasional C driver compile failure.

In addition, we should add an Evergreen builder to the 3.1-dev project that always builds against the tip of the C driver master branch, to ensure that we're up to date with any API changes.