A few improvements to coverage stuff.

* Add coveralls stats to the README.
* Change `ready` script to run using scoverage.
* Fail the tests if minimum coverage isn’t met

The minimum coverage is currently 57%. Please try to aggressively bump
this up in any PR that you can. Since we currently can’t do coverage on
integration tests, changes to the REPL, evaluator, etc. may cause
coverage to drop, but this will require lowering the minimum % in
build.sbt, so we can keep an eye on when that happens.

Also note that scoverage and coveralls percentages won’t match
perfectly. scoverage gives us more correct numbers (statement coverage)
whereas coveralls numbers are based on lines that are touched at
all. The percentage in build.sbt should be relative to scoverage’s
number, which is displayed at the end of the tests:

```
[info] [scoverage] Reports completed
[info] [scoverage] Coverage is above minimum [57.41% > 57.0%]
[info] [scoverage] All done. Coverage was [57.41%]
```