Provide default values for Circle CI environment variables
Provide default values for Circle CI environment variables

This enables running all Makefile rules locally

gshaviv/XCDYouTubeVideoPlayerViewController,0xced/XCDYouTubeKit,mingsai/XCDYouTubeVideoPlayerViewController,gshaviv/XCDYouTubeVideoPlayerViewController,morrisirrom/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,itamaker/XCDYouTubeKit,creationst/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,twobitlabs/XCDYouTubeKit,mingsai/XCDYouTubeVideoPlayerViewController,0xced/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,creationst/XCDYouTubeKit,creationst/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,twobitlabs/XCDYouTubeKit,itamaker/XCDYouTubeKit,twobitlabs/XCDYouTubeKit,mingsai/XCDYouTubeVideoPlayerViewController,creationst/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,morrisirrom/XCDYouTubeKit,0xced/XCDYouTubeKit,0xced/XCDYouTubeKit,mingsai/XCDYouTubeVideoPlayerViewController,twobitlabs/XCDYouTubeKit