An optimized version of solution one to problme 7.

It adoptes a similar approach, as solution 2, in finding a prime larger
than 3: a prime is 6k +/- 1 ( k >= 1 )
The most significant improvement is that, instead of keeping all the primes,
it only keeps a list of the necessary primes which are smaller than the
square root of current N, except the last one in the list.

By eliminating the effort to construct the list of primes on each check,
new solution one is slightly more efficient than solution two.