Change Ruby compiler to jemalloc

[bosawt]

From the writeup here: department-of-veterans-affairs/vets-api#5382

I haven't personally seen the memory footprint of our existing application, but I noticed we weren't using jemalloc flag in our version of Ruby.
( How do I know? If you try this one liner bundle exec ruby -r rbconfig -e "puts RbConfig::CONFIG['MAINLIBS']", you should see -ljemalloc as a config option)

jemalloc is an alternate implementation of the underlying malloc memory allocation function that Ruby uses, and it tends to work quite well with monolith application implementations. I've personally used it on two monoliths and seen dramatic memory footprint reductions, and no downsides. Here is an article describing someone's experience adding it, with a couple tasty graphs:
https://medium.com/rubyinside/how-we-halved-our-memory-consumption-in-rails-with-jemalloc-86afa4e54aa3

If you need further proof, Mike Perham himself tried to make this default in Ruby:
https://bugs.ruby-lang.org/issues/14718
Looks like they were close to adding this into the default, but so far have not agreed on implementing it since it will effect such a wide range of machines. Shouldn't be an issue on our end, especially if we test it properly beforehand.

[bosawt]

@nsoutouras I still want to look into this some day. This is not technically in the purview of the Identity team, but would help the overall backend application footprint. This is completely opportunistic work and has very low priority for us.