Revert "Merge remote-tracking branch 'origin/remove_sdl1_support'"

This reverts commit ffcef6164c079949a7c65674374ec340079d0e01, reversing
changes made to f96c02cf3cf35cc5a5a54eb84cb5efaed8c0f4c6.
Revert "Update travis to use SDL2"

This reverts commit 9dacad174618391ad4c11d60c4e6f7b9b7d9efc7.