Transpile to ES5 code to prevent errors in older browsers

[wbobeirne]

From monero-project/monero-site#777 (comment). Tracked it down to this code:

MoneroVision/src/components/tables/blocks/index.tsx

Line 38 in 44982cc

public componentWillMount() {

. While most users should be on newer browsers, nothing about MoneroVision inherently requires newer browser features, so the code should be as supportive as possible.