Add Xcode 6 beta 1 XCTest to framework search path

After installing Xcode 6 Beta 2, `$(DEVELOPER_FRAMEWORKS_DIR)` (i.e.:
`/Applications/Xcode6-Beta2.app/Contents/Developer/Library/Frameworks/`)
no longer contains XCTest.framework.

Add `$(SYSTEM_APPS_DIR)/Xcode6-Beta.app/Contents/Developer/Platforms/{{OSX, iOS}}.platform/Developer/Library/Frameworks`
to frameworks search path in order to allow Quick and Nimble to find the
"old" XCTest.

Fixes issue #50.