Use real Zeppelin (instead of fork) as E2E

[cgewecke]

Closes #555

This should seamlessly become the Hardhat E2E when they go up.

Notes

• Their codecov command is exiting 0 without uploading reports (good)
• Initially installed with yarn (ignoring the package-lock) and the test run took twice as long (20m vs 10m). Something's going on there maybe.

[codecov-io]

Codecov Report

Merging #557 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #557   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files          20       20           
  Lines         824      824           
=======================================
  Hits          818      818           
  Misses          6        6           

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