fix(yarn): Filter out pure-lockfile option from yarnrc

A repository with .yarnrc specifying pure-lockfile was preventing our yarn from saving a lockfile too! Now filter it out before writing the yarnrc to our tmp directory.

Closes #507

lib/workers/branch/yarn.js

@@ -27,7 +27,11 @@ async function generateLockFile(
       fs.writeFileSync(path.join(tmpDir.name, '.npmrc'), npmrcContent);
     }
     if (yarnrcContent) {
-      fs.writeFileSync(path.join(tmpDir.name, '.yarnrc'), yarnrcContent);
+      const filteredYarnrc = yarnrcContent.replace(
+        '--install.pure-lockfile true',
+        ''
+      );
+      fs.writeFileSync(path.join(tmpDir.name, '.yarnrc'), filteredYarnrc);
     }
     logger.debug('Spawning yarn install');
     // Use an embedded yarn