Add fixed version information to l3build files

As the .dtx and .lua file need to have identical version numbers, using SVN
keyword expansion is not reliable (the same reasoning applies here as to 
expl3.dtx). Thus hard-code the version number/date into the two files, and
for CTAN releases use the same approach as for l3kernel/packages/experimental:
a 'version step' checkin just before release.

Arguably, l3build could have a 'proper' version string (v1.0 at this stage?),
but perhaps it's easiest for the moment just to stick to the SVN revision and
see if we have bug reports. Once things to look 'release quality' then I will
probably change from the SVN number here to something more 'standard'.

Barring any last-minute issues I intend to submit a first release of l3build
to CTAN later today.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5137 de43f980-851b-0410-b2f7-c40aca1f87e0