Properly detect Asterisk instances built from Subversion

Asterisk from SVN uses a different version string to indicate
that it is not from a release tarball.

This commit changes a simple string comparison to a regular
expression check that will match both:

    Asterisk 1.8...
and Asterisk SVN-branch-1.8...
Merge pull request #25 from seanbright/master

Properly detect Asterisk instances built from Subversion