Upgrade SSL lib to avoid deprecated problem

If removed `-Xfatal-warnings` from settings.gradle, `WskCliActivationTests` test case would be successful.
But if kept `-Xfatal-warnings` in settings.gradle, `WskCliActivationTests` test case would be failed. e.g.
```
./gradlew --console=plain :tests:test --tests="system.basic.WskCliActivationTests"
> Task :tests:compileTestScala
[Error] /home/travis/gopath/src/github.com/apache/openwhisk-cli/tests/src/test/scala/system/basic/HttpProxy.scala:97: object AkkaSSLConfig in package akka is deprecated (since 2.6.0): Use Tcp and TLS with SSLEngine parameters instead. Setup the SSLEngine with needed parameters.
[Error] /home/travis/gopath/src/github.com/apache/openwhisk-cli/tests/src/test/scala/system/basic/HttpProxy.scala:102: constructor HttpsConnectionContext in class HttpsConnectionContext is deprecated (since 10.2.0): prefer ConnectionContext.httpsClient or ConnectionContext.httpsServer
two errors found

> Task :tests:compileTestScala FAILED
```
Anaway, `-Xfatal-warnings` (Warnings as errors) is userful, so we should upgrade the lib to fix it with `-Xfatal-warnings`, refer to: https://github.com/apache/openwhisk-cli/pull/517#discussion_r957031960