INTSCALA-58 - Polish build.gradle

* Convert build.gradle from spaces to tabs
* Re-organize repository declarations
* Update and polish the Gradle Maven plugin usage
* Update Gradle Wrapper to Gradle `1.6`
  - The addresses: INTSCALA-53 Upgrade to Gradle `1.6`
* Set source compatibility to `1.6`
* Update Junit dependency to `4.11`
* Update Log4j dependency to `1.2.17`
* Update Mockito version to `1.9.5`
* Update Spring Integration dependencies to `2.2.4.RELEASE`
  - This addresses: **INTSCALA-52** Upgrade spring integration scala dsl to the latest release version of Spring Integration.
* Update Spring Gemfire dependency to `1.3.0.RELEASE`
* Fix **INTSCALA-51** - Invalid sources artifacts in maven repository
  - This also addresses: **INTSCALA-27** Fix build to ensure that sources are packaged and published.
* Update used Scala library to `2.10.2`
* Update Jackson dependency to `1.9.12`
* Update test dependency Derby to `10.10.1.1`
* Update test dependency ActiveMQ to `5.7.0`
* Rename several private traits due to Scaladoc collisions
  - In SftpDsl rename trait WithOutboundAttributes to WithSftpOutboundAttributes
  - In FtpDsl rename trait WithOutboundAttributes to WithSftpOutboundAttributes
  - In FtpDsl rename trait WithInboundAttributes to WithFtpInboundAttributes

For reference see: https://jira.springsource.org/browse/INTSCALA-58