fix(yarn): spawn in shell on Windows (#138)

Must spawn `yarn` within a shell on Windows for the `yarn`
executable to be resolved correctly. Otherwise a ENOENT
error is received because the `yarn` command can't be found.

Spawning a comamnd in a shell on Linux should behave the same,
though there may be a small increase in time required to
spawn the process.

lib/helpers/yarn.js

@@ -20,7 +20,7 @@ async function generateLockFile(newPackageJson, npmrcContent, yarnrcContent) {
     if (yarnrcContent) {
       fs.writeFileSync(path.join(tmpDir.name, '.yarnrc'), yarnrcContent);
     }
-    cp.spawnSync('yarn', ['install'], { cwd: tmpDir.name });
+    cp.spawnSync('yarn', ['install'], { cwd: tmpDir.name, shell: true });
     yarnLock = fs.readFileSync(path.join(tmpDir.name, 'yarn.lock'));
   } catch (error) {
     tmpDir.removeCallback();