build(npm): lock systemjs at 0.18.10 due to regressions in future versions

otherwise in dist/js/dev/es5/benchmarks/src/naive_infinite_scroll/index.html
defaultExtension: 'js' is ignored for angular2/src/testing/benchmark_util which
results in test failures.

We'll need to investigate this. Maybe upgrading to 0.19.x will make this issue go
away...

package.json

@@ -103,7 +103,7 @@
     "source-map": "^0.3.0",
     "strip-ansi": "^2.0.1",
     "symlink-or-copy": "^1.0.1",
-    "systemjs": "^0.18.10",
+    "systemjs": "0.18.10",
     "systemjs-builder": "^0.10.3",
     "through2": "^0.6.5",
     "ts2dart": "^0.7.12",