Allow JEMALLOC_VERSION to change version without code change

[gaffneyc]

It has come up a couple times that people are surprised that just changing JEMALLOC_VERSION isn't enough. Today, changing the environment variable also requires a code push because it is copied in during the build phase. This helps avoid applications issues if the version of jemalloc changes or isn't available during a dyno reboot. It also means we're much better citizens to GitHub since we make one request per build instead of one request per restart per dyno.

To make this work we would need to do a version check during the boot process. Heroku offers some storage for buildpacks where we could stash downloaded versions instead of always requesting them from GitHub, or only need to request them once per version.