This skips version 3.0.2 and goes directly to 3.1.1. There are no
compilation issues and the apache-spark benchmarks execute normally.

SBT uses 1024 by default and kills memory-related JVM options in the
`.sbtopts` file and derives its values on its own. The default memory
size results in code cache of 128 megs, which is insufficient for a
pristine build, where it can reach slightly over 200 megs. Raising the
memory to 2048 increases the code cache size to 256 megs.

This will avoid having to change the version number in multiple
places. The current version number is determine by the latest
(annotated) tag and the distance to it from the current commit.
Release built from the tagged commit will have a "clean" version
number, others will include the hash of the build commit.

CI scripts will be able to determine the current version simply
by using `git describe` and stripping the initial `v`.

The only thing that needs to be updated is the git.baseVersion
setting in renaissance-core/version.sbt, which provides a base
(upcoming) version number for git-less situations, i.e., when
building the sources outside the git tree.

This will avoid having to update README.md with each commit.

Removes task to write out .gitprops which does not work in git-less
mode and is not needed anyway. Also adds a proper version related to
the upstream master and the exported dependency.

These checks are also made in the CI, but it does not hurt to
have them in the pre-push hook. The markdown check requires a
successful build of the base JAR.

The goal is to use the `script` section of `.travis.yml` for
higher-level actions instead of low-level file management.