Fix validate_attestation_slot

[hwwhww]

What is wrong?

https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#attestations-1

1. Add assertion for Verify that attestation.data.slot >= GENESIS_SLOT

    # Can't submit attestations that are too far in history (or in prehistory) 
    assert attestation.data.slot >= GENESIS_SLOT
    assert state.slot < attestation.data.slot + SLOTS_PER_EPOCH
    # Can't submit attestations too quickly
    assert attestation.data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot

2. Fix other assertions around MIN_ATTESTATION_INCLUSION_DELAY (Fix slot range for attestation inclusion consensus-specs#669)

How can it be fixed

Fill this in if you know how to fix it.