If svn does not exist, fall back to /usr/bin/svnlite in lib/subversion.sh

[jbirch]

For the two operations being performed -- update and info -- it's a drop in replacement. I've a symlink from /usr/local/bin/svn -> /usr/bin/svnlite and Crochet works as expected.

This change would remove one further dependency or fudged-step from a bare system being able to create images with Crochet.

[kientzle]

Updated lib/scm.sh to try 'svn' and fall back to 'svnlite' if that fails.

Please try and let me know how it works for you.

[jbirch]

I think that'd be fine; I'll give that a try later this week.

One thing -- I think this'll print svn: not found or equivalent in the case that svn isn't actually on the system. Using which will allow us to get around this -- I'll raise a PR later in the week when I can test it.

Thanks!