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.

I don't know if NTstation has a head maintainer or if there are head maintainers, but they should probably be the ones to make a descision about this.

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