Upgrade to latest akka 2.6.16

[mkurz]

Also fixes #601

[mkurz]

Actually there is a problem with the travis config, see https://app.travis-ci.com/github/playframework/play-ws/jobs/532586654#L229

$ export TRAVIS_SCALA_VERSION=2.12.14
...
$ sbt ++$TRAVIS_SCALA_VERSION 'testOnly -- xonly timefactor 5'
Detected sbt version 1.5.5
Using jvm options defined in file /etc/sbt/jvmopts
# Executing command line:
java
-Xms2048M
-Xmx2048M
-Xss6M
-XX:MaxPermSize=512M
-jar
/home/travis/.sbt/launchers/1.5.5/sbt-launch.jar
++2.13.6                                                                <<<------------- WHY?????
"testOnly -- xonly timefactor 5"

It runs with scala 2.13.6 instead of 2.12.14

[SethTisue]

@mkurz I redid the Travis-CI config. but on 2.12 there's still a 0.8 vs 1.0 problem with scala-java8-compat

[mkurz]

Green after dropping Scala 2.12 🚀