perf: Bump light.js to use getTxByHash

[amaury1093]

This requires parity-ethereum 2.5.0. Let's wait for it to be released, and test this.

Implements openethereum/js-libs#213

[Tbaut]

I've tested it on top of the security PR #451 the following way:

• Merged the security branch and this branch locally.
• I commented the postinstall script that checks and download parity in fether-electron/package.json because Parity Ethereum v2,5 isn't released yet.
• checkout beta from Parity Ethereum (v2.5)

- cherry-picked the commit responsible for the improvement (openethereum/parity-ethereum@10e1787)
git cherry-pick 10e1787ad1aa589912991c90d65f356a097d98a7 edit: not needed anymore

• Built and launched Parity Ethereum 2.5

Worked well for normal and Signer accounts

[amaury1093]

I just tried with yarn install && yarn electron, everything works: 2.5.0 was downloaded, and I could send a tx (which polled getTxByHash).