Make build more portable.

- Explicitly set sbt version instead of guessing from launcher.

- Remove Scala cross-built dependencies that fail without explicit
  Scala version.  If the project decides to use them, it should set an
  explicit scalaVersion in the build.
Port log4j to slf4j.

This allows the library be used in projects that use an alternate
logging backend, such as logback.  log4j is retained as the logging
backend for tests.
Merge pull request #18 from CrowdStrike/develop

Use slf4j instead of log4j to support other logging backends
Changes getSpecAndRoot(..) as per Oscar
Merge pull request #20 from krishnanraman/develop

Changes getSpecAndRoot(..) as per Oscar
Merge branch 'release/1.3.2' into develop
Merge branch 'develop' of github.com:nathanmarz/dfs-datastores into develop
version bump
Merge branch 'release/1.3.3' into develop
keep around a configurable number of versions, default 3
uncommited fixes and add versionedstore unit test
Merge pull request #24 from boazavital/keepversions

keep around a configurable number of versions, default 3
bump
Merge branch 'release/1.3.4' into develop
Adding support for snappy.
Merge pull request #26 from fkautz/develop

Adding support for snappy.
bump snapshot, implement getModifiedTime for cascading 2.1
Revert "Merge pull request #26 from fkautz/develop"

This reverts commit 06b8fa371f36091228741c999441a02d72132dcd, reversing
changes made to 2fd285d33713cf636743fb115d3813ca14049ae5.
throws
Changes to VersionStore.