Fix Travis after LLVM APT repo change
Fix Travis after LLVM APT repo change

After LLVM 6.0 was branched, the LLVM APT repositories changed so that
clang-format-7 is in the "latest" repo instead of 6.0. We cannot use
the LLVM-6.0 repo in Travis yet because it isn't in the repo whitelist.
Update travis.yml to use clang-format-7

Kangz/nxt-standalone,googlearchive/nxt-standalone,Kangz/nxt-standalone,Kangz/nxt-standalone,googlearchive/nxt-standalone,Kangz/nxt-standalone,googlearchive/nxt-standalone,googlearchive/nxt-standalone