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 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