Bump version number to 4.2.0

There have been plenty of nice improvements in the last time, and it's
been over 14 months since the last release! Let's cut a new release to
get these out.

Mention CLCACHE_MEMCACHED environment variable in README

Bump version number to 4.0.0

A couple of rather big improvements to performance were applied; greatly
improved concurrency with multiple clcache processes, better handling of
invocations with multiple source files, an optional caching server for
hash sums. The diff since 3.1.1 is quite large already, let's do a major
release.

Bump to version 3.3.1

We have a few smaller improvements and bugfixes, let's do a release!

Bump version to 3.3.0

Releasing clcache 3.2.0

Bump version number to 3.1.1

Bump version number to 3.1.0

Bump version to 3.0.2

The configurable cache lock timeout, the direct mode fixes and a few
small things like the prettier 'stats' output are all nice user-visible
improvements which justify doing a bugfix release.

Version 3.0.1

The stdout fix and the undefined identifier fix are nice bug fixes,
so let's do a release.