Add rails 5.2 and ruby 2.5.3 compatibility

[lleirborras]

Tests pass on my local environment

[coveralls]

Coverage decreased (-0.1%) to 99.895% when pulling b6a507e on feature/rails5.2 into aa0c85d on master.

[coveralls]

Coverage decreased (-0.1%) to 99.895% when pulling b6a507e on feature/rails5.2 into aa0c85d on master.

[coveralls]

Coverage decreased (-0.1%) to 99.895% when pulling b6a507e on feature/rails5.2 into aa0c85d on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 33cc4f9 on feature/rails5.2 into aa0c85d on master.

[coveralls]

Coverage decreased (-0.1%) to 99.895% when pulling b6a507e on feature/rails5.2 into aa0c85d on master.

[coveralls]

Coverage decreased (-0.1%) to 99.895% when pulling b6a507e on feature/rails5.2 into aa0c85d on master.

[coveralls]

Coverage decreased (-0.1%) to 99.895% when pulling b6a507e on feature/rails5.2 into aa0c85d on master.

[coveralls]

Coverage decreased (-0.1%) to 99.895% when pulling b6a507e on feature/rails5.2 into aa0c85d on master.

[vjt]

This has been green in production since quite some time.