test(*): improve travis config
test(*): improve travis config

Node.js 7.10.0 only fails in the general tests, not in the dependency
tests, so there's no reason in testing dependencies for both 7.10.0 and
7.9.0.

And for now let's also stop testing 7.10.0 in the general tests as it's
not likely that the 7.x branch will ever get fixed :(

opbeat/opbeat-node,opbeat/opbeat-node