gnosis / dex-services / 5471

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travis-pro

Make gas estimators configurable (#1524)

This is useful in case an estimator becomes broken and we want to
disable it without recompilation.

I set the default estimator to web3 because this is the only one that works on every network.

This is a config change because we do not want to use the new default.

### Test Plan
Ran the binary locally to make sure arguments are interpreted correctly.

14056 of 56163 branches covered (25.03%)

Branch coverage included in aggregate %.

3771 of 5888 relevant lines covered (64.05%)

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