Upgrade SSL lib to avoid deprecated problem

[ningyougang]

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: #517 (comment)