Clock: Add deriving Eq to stub to make it testable

[sshine]

Clock is the last remaining exercise that has the .meta/DONT-TEST-STUB file because the type in the src/Clock.hs stub is not compatible with the test suite.

Other examples of exercises that were revised so that testing the stub became possible are listed in #466. The difference is that those all had data definitions, whereas Clock is ideally solved with a 'newtype' and an appropriate alias.

The DONT-TEST-STUB mechanism was added by @petertseng in #464 and in particular the commits

• 1d91ceb (Travis code)
• 342b932 (Clock-specific)

The change made in this commit argues that since the example solution has deriving Eq, adding this to the stub will make the exercise a bit easier, but it still won't give away the central piece.

The motivation behind removing DONT-TEST-STUB in Clock is that with this single effort, we can say that all stubs can be compiled and tested. To motivate further, I will cite @rbasso in #466:

I think that this is the way to go. We have to sacrifice a little
flexibility in designing exercises, but we gain a more coherent
user-experience and automatic testing of stubs. 👍

This bumps the exercise's version from 2.4.0.9 to 2.4.0.10.

[sshine]

Additionally, I have two notes for reference:

• We can still, for now, keep the functionality in .travis.yml.
• In the context of adding "concept exercises" for "Exercism v3", we can simplify the union of the current track repo's directory structure and what is proposed as the future directory structure once "concept exercises" become a thing. (In regards to the many consequences that that pull request would have on this repo, I have remarked there that I intend to create issues in this repo's issue tracker discussing each point.)

[petertseng]

wow, we must have completely forgotten to do this when doing #695 . The DONT-TEST-STUB file mentions the Num instance, but #695 has told us that the Num instance is a bad idea. Adding deriving Eq to the stub to make it testable is eminently reasonable and should have been done a long time ago, so this PR is just doing what we forgot to do back then.

[sshine]

@iHiD: I'm a bit unsure why this happens:

Could it be related to my recent permission changes in relation to the exercism/v3 repository?

[petertseng]

I would like to note for the record that I am also not authorised, but that I assume this problem will be solved for the entire Haskell maintainer team at once, so perhaps I need not have mentioned it.

[iHiD]

Should be fixed for this repo now. It's currently broken everywhere.