Fix bug in benchmark

Since previous commit, Acorn is no longer properly benchmarked
when locations are on.

packages/babylon/test/bench.html

@@ -35,7 +35,7 @@ <h1>Acorn/Esprima/UglifyJS speed comparison</h1>
 
 <script>
   function runAcorn(code, locations) {
-    acorn.parse(code, {linePositions: locations});
+    acorn.parse(code, {locations: locations});
   }
   function runEsprima(code, locations) {
     esprima.parse(code, {loc: locations});