Upgrade to pex 0.8.4 to pick up perf wins.
Upgrade to pex 0.8.4 to pick up perf wins.

Testing Done:
Basic perf test shows a nice win for interactive use:

Old
===
2nd+ consecutive runs
time pants.dev test tests/python/pants_test/engine
...
real	0m5.989s
user	0m2.897s
sys	0m0.453s

New
===
2nd+ consecutive runs
time pants.dev test tests/python/pants_test/engine
...
real	0m1.973s
user	0m1.350s
sys	0m0.220s

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/48943230

Bugs closed: 1014

Reviewed at https://rbcommons.com/s/twitter/r/1693/

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