Support Xcode 8 GM

* Drop test coverage for Xcode 7 since it's likely not installed
* We now pull paths from xcode-select when we don't specify a toolchain which is saner than using hardcoded paths
* We now more robustly test if a given toolchain is from xcode 7 or not rather than rely on the filename
* Bootstrap for Xcode 8 installed to Xcode.app
2 jobs from pull/119/head in 5 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #5715
autoscale-linux
linux

05:20

passed #5716
openswift xcodeselect8
osx

02:14