Re-enable Truffle E2E test

[cgewecke]

This is a tracking issue to make sure we don't forget to begin running the E2E test which was built to validate each pull request against Truffle tsc builds and contract tests.

As noted in #3304, Truffle has reverted to 1.2.1 because truffle-contract needs changes to its emitter wrapping to correctly handle the confirmations logic fixed in 1.2.2. (See truffle 2688)

We cannot inject 1.2.2 and above into their test suite because the 1.2.1 and 1.2.2 types are managed differently there and tsc fails on install.

[cgewecke]

Closing, we've added two additional E2E targets which cover +/- the same ground.

• gnosis/dex-react (TS compilation)
• mosaic (@truffle/contract unit tests)

Truffle is locked on 1.2.1 and there's no movement over there.