Add support to fail build if coverage drops since last build

[deepujain]

If you store coverage number for previous builds and fail build if the number of lower. This will ensure that every commit does not lower the coverage and is easy to hold a specific developer responsible for lowering the coverage so that test cases could be added.

What you think.

[shanbin]

I'd recommend using scoverage SBT plugin to achieve this, https://github.com/scoverage/sbt-scoverage#minimum-coverage, then we will have consistent behavior between local builds and Jenkins builds.

[deepujain]

I do use minimum-coverage but I need to manually keep increasing the limit. And it does not rigorously allow to maintain coverage with every commit.