Use require.resolve() to get the path of testrpc-sc

lib/app.js

@@ -176,11 +176,11 @@ class App {
       if (!this.norpc) {
         const defaultRpcOptions = `--accounts ${this.accounts} --port ${this.port}`;
         const options = (this.testrpcOptions || defaultRpcOptions) + ` --gasLimit ${gasLimitHex}`;
-        const command = './node_modules/ethereumjs-testrpc-sc/build/cli.node.js ';
+        const command = require.resolve('ethereumjs-testrpc-sc/build/cli.node.js');
 
         // Launch
         const execOpts = {maxBuffer: 1024 * 1024 * 10};
-        this.testrpcProcess = childprocess.exec(command + options, execOpts, (err, stdout, stderr) => {
+        this.testrpcProcess = childprocess.exec(`${command} ${options}`, execOpts, (err, stdout, stderr) => {
           if (err) {
             if (stdout) this.log(`testRpc stdout:\n${stdout}`);
             if (stderr) this.log(`testRpc stderr:\n${stderr}`);