Fixed bootstrap test for newer setuptools.

setuptools used to say this:

    No local packages or download links found for zc.buildout==UNKNOWN

Since setuptools 25.1.0 it says this:

    No local packages or working download links found for zc.buildout==UNKNOWN

See also https://github.com/pypa/setuptools/pull/609/files

src/zc/buildout/bootstrap.txt

@@ -70,7 +70,7 @@ Let's try with an unknown version::
     ...
     X
     ...
-    No local packages or download links found for zc.buildout==UNKNOWN
+    No local packages or...download links found for zc.buildout==UNKNOWN
     ...
 
 Now let's try with `2.0.0`, which happens to exist::