Add test for truffle-migrate example

[spalladino]

The truffle-migrate example is not currently being compiled nor tested in the CI, though testing it should be straightforward by running the commands listed on its README in a bash script that is exercised by the CI.

To run this project locally, first install all dependencies with npm install, and then start a ganache instance by running npx ganache-cli -p 9545. Next, run all migrations via npx truffle migrate --network local. Finally, check the state of the deployment via npm run check.

The only change required should be modifying index.js (the script called as part of npm run check) to throw (and thus have a non-zero exit code) if the value returned is not the expected one (52 IIRC).

This test should be somehow added to .travis.yml as another stage to be run.