PListImplTest fails on AIX and Solaris

PListImplTest.testConcurrentAddRemove fails on both AIX and Solaris. I think this is just a timing issue, because both of these boxes are really slow. However, my initial brute force effort to fix this by just increasing the timeout somehow caused the test to fail on other platforms.

For now I've added an exclude to the pom so the other tests will get run on these platforms.