exclude saxon version from checkstyle

Description:

as xwiki participate in checkstyle's regression, we use checkstyle snapshot version to validate xwiki code.

Unfortunately there are bunch of same dependecies are used by checkstyle and xwiki, so it creates problems during build.

example: https://github.com/checkstyle/checkstyle/pull/7349

example of build failure: https://app.wercker.com/checkstyle/checkstyle/runs/build/5dededd039029f0008eaa1d5?step=5dedee1aa25c6b0008b27feb

example of discussion to exclude some other dependency - https://github.com/xwiki/xwiki-commons/pull/76/commits/9182aa3cfbd647b0fa026f4e18892a7f49963f0e to avoid the same problem. But Saxon is very frequent released artifact, so problem will acute frequently.

Expected:

it would really helpful to exclude such dependency from checkstyle in pom.xml to let use only version from xwiki code.