Upgrade Rails to 4.2.10

[maciej-szlosarczyk]

#546 is blocked by this one.

To get to the latest Ruby version that comes with security patches, we need to upgrade Rails to at least 4.2.10. Otherwise we'll run into Fixnum/Bignum unification issues. I tested it via docker, the test suite does not even run due to that bug:

rails/rails#25125

Since this is a point release, the actual upgrade should be relatively painless.