Plug with Travis CI service

Following customizations are required to get ~stable builds on travis
cloud workers:

- Avoid test timeouts with akka.test.timefactor=1.5
- Avoid memory problems with smaller thread stack size (Xss2m)
- Avoid non-deterministic timing/perf issues by serializing the tests
  (set concurrentRestrictions in Global += Tags.limit(Tags.Test, 1))