Configure isolated travis build for kohana/core

This is the first of the changes to add separate CI support for each module in isolation as discussed in kohana/kohana#50.

Once this is done and working across all the 3.3 modules, I'll merge up to 3.4 and make any required changes for that.

@shadowhand, @zombor or @Zeelot I think only you (as admins) can activate the travis hooks? Could you do that now and then I'll force a push to get the build running? I think you could activate for all the core modules now, obviously all the others will fail for the minute as they don't have travis configuration but I should get to them pretty quickly.