Put ARTIFACTS_VERSION in a config

packages/0x.js/test/global_hooks.ts

@@ -13,6 +13,6 @@ before('migrate contracts', async function() {
         gas: devConstants.GAS_ESTIMATE,
         from: devConstants.TESTRPC_FIRST_ADDRESS,
     };
-    const artifactsDir = '../migrations/artifacts/1.0.0';
+    const artifactsDir = `../migrations/artifacts/${constants.ARTIFACTS_VERSION}`;
     await runMigrationsAsync(provider, artifactsDir, defaults);
 });

packages/0x.js/test/utils/constants.ts

@@ -6,4 +6,5 @@ export const constants = {
     KOVAN_RPC_URL: 'https://kovan.infura.io/',
     ROPSTEN_RPC_URL: 'https://ropsten.infura.io/',
     ZRX_DECIMALS: 18,
+    ARTIFACTS_VERSION: '1.0.0',
 };