Defines corejs v2 in the gulpfile

[nivida]

Description

The corejs options property was missing in the babel config and brought up a warning during bundling of web3.js. Instead of letting babel choose automatically version 2 of it do we now defined it explicit.

Type of change

• Enhancement

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test with success and extended the tests if necessary.
• I ran npm run build and tested the resulting file from dist folder in a browser.
• I have tested my code on the live network.

[nivida]

@cgewecke Merged without review because it doesn't change any logic and just defines the anyways chosen setting explicit.