travis, build, consensus: drop support for Go 1.7

consensus/ethash/consensus.go

@@ -53,7 +53,6 @@ var (
 	errDuplicateUncle    = errors.New("duplicate uncle")
 	errUncleIsAncestor   = errors.New("uncle is ancestor")
 	errDanglingUncle     = errors.New("uncle's parent is not ancestor")
-	errNonceOutOfRange   = errors.New("nonce out of range")
 	errInvalidDifficulty = errors.New("non-positive difficulty")
 	errInvalidMixDigest  = errors.New("invalid mix digest")
 	errInvalidPoW        = errors.New("invalid proof-of-work")
@@ -474,18 +473,13 @@ func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Head
 	if ethash.shared != nil {
 		return ethash.shared.VerifySeal(chain, header)
 	}
-	// Sanity check that the block number is below the lookup table size (60M blocks)
-	number := header.Number.Uint64()
-	if number/epochLength >= maxEpoch {
-		// Go < 1.7 cannot calculate new cache/dataset sizes (no fast prime check)
-		return errNonceOutOfRange
-	}
 	// Ensure that we have a valid difficulty for the block
 	if header.Difficulty.Sign() <= 0 {
 		return errInvalidDifficulty
 	}
-
 	// Recompute the digest and PoW value and verify against the header
+	number := header.Number.Uint64()
+
 	cache := ethash.cache(number)
 	size := datasetSize(number)
 	if ethash.config.PowMode == ModeTest {