Add formula for qt 4.8 and update coinbase dependency

[coagmano]

Copying cartr/homebrew-qt4 both as a backup and so Gridcoin users on Mac don't have to tap multiple repos

As mentioned here: https://cryptocointalk.com/topic/13139-os-x-builds-feedback-bug-reporting/page-14

[coagmano]

hmm timed out while building QT because there's no qt@4 bottle yet. I'll rename it back to qt so it can retrieve the existing bottle for 4.8

[ghost]

Nice job. Thanks!