Updates BYOND_MINOR and BYOND_MAJOR vars in .travis.yml

Updates the BYOND_MINOR and BYOND_MAJOR environment variables in .travis.yml, so Travis will start compiling with BYOND 506.1247 instead of BYOND 503.1224.
Merge pull request #4251 from JJRcop/patch-1

Updates BYOND_MINOR and BYOND_MAJOR vars in .travis.yml