Java 1.7u51

Since Java is being updated in  phinze/homebrew-cask#3601 to version 8, I'm moving version 7 here.

I'm wondering if it's a good idea to have two Java7 casks: `java7u45` and `java7u51`, wouldn't it be better to have just one `java7` cask with the latest version of Java since 45 and 51 are just minor updates? Just like Java6, one cask.

If you prefer to have just one Java7 cask, tell me and I'll update this pull request.