Fix miniumum JDK version in README.md

[yosske]

The "Building Notes" in the README.md file says the minumum JDK for building is:

"JDK 7"

Instead Of:

"JDK 8"

JDK 8 is needed ever since this fix was done:
https://github.com/google/openrtb/blob/master/CHANGELOG.md#version-097-29-10-2015

Thanks :)

[opinali]

Thanks for the heads up, fixed the readme to mention correct versions for JDK and also protoc. :)