Auto merge of #3731 - phansch:travis, r=phansch

Auto merge of #3731 - phansch:travis, r=phansch

Travis: Don't run integration tests on every PR commit

This does not save Clippy any time but it makes sure that the concurrent
build limit is not reached as quickly for the `rust-lang` Travis
account.

I can't create a permalink to the discussion somehow, so here's an
excerpt from the Infra channel:

```
[11:57 PM] pietroalbini: there is a clippy build (20 jobs) and a packed_simd one (42 builders)  and a rustc one which isn't scheduling atm
[11:58 PM] pietroalbini: I don't think there is a way to prioritize rustc builds in the queue on travis, right?
[12:22 AM] alexcrichton: pietro: I don't think so no
[12:22 AM] alexcrichton: If it's a problem we should cull builds on other projects
[12:22 AM] alexcrichton: The rust repo is the #1 priority
```

Since the integration tests are rarely failing these days, I think it's
fine to not run them on every commit.

If needed, it's also still possible to do a complete test run with `@bors try`.