explicitly invalidate cache key when transform options change

[jwbay]

Transform cache invalidation for coverage seems to implicitly rely on instrumentation changing some value in tsconfig. Besides making the invalidation more explicit, since the last value passed to getCacheKey is an options bag, ts-jest can insulate itself from new transform options being added in the future that should also bust the cache by just stringifying the object. I'm trying to get a change into jest that adds mapCoverage to the options here, for example.

[kulshekhar]

@jwbay thanks for the PR :)

Can you also update the authors file with your name?

This looks good to me and can be merged in after one other collaborator okays it.