Increase code test coverage

[jdesiloniz]

Based on current reports from codecov.io (https://codecov.io/gh/scala-exercises/scala-exercises/branch/master), the current code test coverage in the project is near 20%. A minimum value should be set (i.e. cats library - https://github.com/typelevel/cats - has this minimum set to 60%), increase the test coverage to at least that value and once it's covered, enforce it through the scoverage settings (ScoverageKeys.coverageMinimum and ScoverageKeys.coverageFailOnMinimum)