Update JsonpExportMainTemplatePlugin.js

Fix:

`TypeError: Object #<JsonpExportMainTemplatePlugin> has no method 'applyPluginsWaterfall'`

This is the same bug as was fixed in https://github.com/webpack/webpack/commit/614891e
Merge pull request #529 from plaxdan/master

JsonpExportMainTemplatePlugin has no method 'applyPluginsWaterfall'