Updates .travis.yml BYOND_MINOR env var to "1247" [DON'T MERGE YET]

Updates the BYOND_MINOR environment variable in .travis.yml, so Travis will start compiling with BYOND 503.1247 instead of BYOND 503.1224.

### DISCUSSION REQUIRED

* Is BYOND 503.1247 stable enough to be compile-testing with, or should we stick with BYOND 503.1224?