Refrain from starting ganache when targeting a testnet

[geoknee]

The whole point of pre-deployed-artifacts is to avoid the need to deploy contracts (especially on test or mainnet where this is likely to be slow).

You'll notice that running TEST_NETWORK=ropsten yarn start results in ganache starting, and contracts being deployed.

[geoknee]

from @lalexgap

I think this is the culprit in the start.js script:

const {
      deployContracts,
      startGanache
    } = require('magmo-devtools');
    let argv = require('yargs').argv;
    startGanache(argv).then(() => {

      deployContracts().then(value => {

          const devServer = new WebpackDevServer(compiler, serverConfig);