In 2005 Cameron told Blair that "He(Blair) was the future once".
When we say someone is the future we mean that that person will determine the future direction of a country/ government/ organisation/ society. There is a common saying that "children are the future". We also might say "Pokemon Go is the future (of computer games)"
Cameron was mocking Blair by saying that he (Blair) had run out of new policies, and would soon be out of office. Implicitly Cameron was telling us that he would be the future, that he would be able to determine the future direction of the country by eventually becoming PM.
Now, as he hands over to May he is turning the line around, as a bit of rhetoric.