fix(effects): make correct export path for testing module

[jeffbcross]

Closes #94

[jeffbcross]

When I diff the dist folder before and after this change, the only changed file is dist/effects/testing.d.ts

Before:
export * from './effects/testing/index';

After:
export * from './testing/testing';

[coveralls]

Coverage remained the same at 88.365% when pulling 986b07c on jeffbcross:fix-testing into 6982952 on ngrx:master.

[jeffbcross]

And I verified that this fixes the error in my application.

[vsavkin]

LGTM