spray-io-tests: improve resiliency of SslTlsSupport tests against `Tcp.Received` message framentation

The issue could produce hanging tests when the SpraySslServer actor died.

refs #233
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Travis CI: Tune JVM args of sbt (Xss3M)

Override default thread stack size (3M instead of 6M)
Try with JVM/SBT opts used by sirthias