fix(contrib): use 1GB per VM

Most modern laptops have ~8GB of memory available to them. On OSX,
this is stretched even thinner due to boot2docker. Lowering the memory
requirement for each host back to CoreOS' default memory requirements
should allow us to use Vagrant on machines with less than 16GB of
memory.
Merge pull request #2746 from bacongobbler/lower-mem-requirement

fix(contrib): use 1GB per VM