Drop support for browser versions older than Chrome 55 and Firefox 52

Background

The minimum version of Google Chrome on which Adblock Plus is officially supported is version 52. This version is more than 3 years old. Similarly, the oldest supported version of Firefox is version 51. If the Adblock Plus team is to drop support for older versions and raise the minimum to Chrome 55 (December 2016) and Firefox 52 (March 2017), it would allow the team to upgrade the Adblock Plus code to use newer features like async/await.

What to change

In metadata.chrome change chrome = 52.0 to chrome = 55.0, and opera = 39.0 to opera = 42.0.

In metadata.gecko change gecko = 51.0 to gecko = 52.0.