build scala-extensions for 2.11

This adds a target for `scala-extensions` to compile with version 2.11. Note that I opted to keep the current build structure of having `scala-extensions-{version}` and just copied the code to get it working, which didn't require any changes. I am usually very reluctant to copy code and the craftsman in me wants me to change the build so sbt will cross-compile the same code for different scala targets, but I assume the structure is there to be able to handle version differences so I went with this one for now. The only change required was to update `com.twitter.util-core` to the latest version, which has a 2.11 build available.