What steps will reproduce the problem?
1. Running the schematron task without saxon defined in the system classpath
See
http://stackoverflow.com/questions/23132201/setting-ant-classpath-in-build-xml
for someone [else]'s description of and proposed solution to the problem.
This is an issue when you can't define the classpath for saxon ahead of running
an ant script.
Moved from Schematron/schematron#13