Unfortunately what we're using right now, Xcode 8.2, cannot compile LLVM for OSX
10.7. We've done this historically and Gecko would like to maintain this
compabitiliby. This commit moves our release builders for OSX to using Xcode 7
which can compile LLVM for 10.7.

The builders running tests continue to use Xcode 8.2, however, because the LLDB
version with Xcode 7, 350, is blacklisted in running our LLDB tests. To continue
running LLDB tests we'll stick with Xcode 8.2.