Verify testrpc-sc location in node_modules

lib/app.js

@@ -128,10 +128,17 @@ class App {
   launchTestrpc() {
     return new Promise((resolve, reject) => {
       if (!this.norpc) {
+        let command;
+
         const defaultRpcOptions = `--gasLimit ${gasLimitHex} --accounts ${this.accounts} --port ${this.port}`;
         const options = this.testrpcOptions || defaultRpcOptions;
-        const command = './node_modules/ethereumjs-testrpc-sc/build/cli.node.js ';
-
+        const npm = './node_modules/.bin/testrpc-sc';
+        const yarn = './node_modules/ethereumjs-testrpc-sc/build/cli.node.js';
+      
+        shell.test('-e', yarn)
+          ? command = `${yarn} `
+          : command = `${npm} `;
+  
         // Launch
         this.testrpcProcess = childprocess.exec(command + options, null, (err, stdout, stderr) => {
           if (err) {