Tune glibc memory usage

There is a report of better memory usage with this setting.

See travis-ci/travis-cookbooks#458 and https://devcenter.heroku.com/articles/testing-cedar-14-memory-use.

Author: BanzaiMan

.travis.yml

@@ -21,6 +21,7 @@ os:
 env:
   global:
     - JAVA_OPTS="-XX:+TieredCompilation -XX:TieredStopAtLevel=1"
+    - MALLOC_ARENA_MAX=2
   matrix:
     - PHASE='-Ptest'
     - PHASE='-Prake -Dtask=test:jruby'