Modify config.yaml of CircleCI to allow rerunning failed tests

[wehs7661]

For unknown reasons, the unit tests for functions that use MPI occasionally fail with uninformative STDOUT/STDERR. Instead of figuring out the underlying reason for these failures, an easier way to ensure that at least the build on the master branch passed the CI may be simply rerunning the CI. However, this would require some modifications in config.yaml. Some instructions can be found here.