Set MALLOC_ARENA_MAX to prune memory use
Set MALLOC_ARENA_MAX to prune memory use

This sets MALLOC_ARENA_MAX, as mentioned by travis-ci/travis-cookbooks#458, to attempt to decrease memory use of tests

kluver/lenskit,kluver/lenskit,kluver/lenskit,tajinder-txstate/lenskit,vaibhav345/lenskit,tajinder-txstate/lenskit,vaibhav345/lenskit,kluver/lenskit,vaibhav345/lenskit,kluver/lenskit,tajinder-txstate/lenskit,tajinder-txstate/lenskit,vaibhav345/lenskit