Bump httpclient in storm-core to 4.2.5.

This PR resolves an issue I came across when attempting use the lastest AmazonS3Client with storm. The httpclient 4.1.1 present on the classpath in the storm cluster caused an exception to occur when attempting a request with the AmazonS3Client. The aws-sdk-java library is currently including httpclient 4.2 (see [here](https://github.com/aws/aws-sdk-java/blob/master/pom.xml#L40)). This PR bumps the httpclient to 4.2.5.