Add a -race enabled build to Travis

[schmichael]

Allow it to fail for now with the goal of making it the default build in
the future.

Thanks to @jippi for calling attention to this in #4608. We should have added this build a long time ago so we could be race-free by now, but better late than never!

(If I get the .travis.yml right on the first try I'm giving myself a 🍪)

[jippi]

Thanks @schmichael - will it be a 0.9 or 0.10 goal? all of those -race issues are imo critical bugs in production software, and should have importance than release schedules or new features - especially since its real issues in prod for me/users

[schmichael]

@jippi We're not going to commit to 0 race detector failures by release X, but race failures in runtime code will be fixed for 0.9. Some of our tests are unfortunately racy in how they peek into internal state, and while we try to fix those as we come to them we're not going to hold up a release for non-runtime bugs.

0 race detector failures and the -race enabled build being the default is definitely our goal though.

[jippi]

@schmichael great - i totally agree with synth vs runtime errors

[schmichael]

(For the record I did manage to get the .travis.yml right on the first try! No cookies around though...)

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