Bump tycho version to 0.18.1, use m2e 1.5

Signed-off-by: Fred Bricon <fbricon@gmail.com>
Fix disappearing classpath entries on project update

The new test case couldn't reproduce the issue, but at least can serve as
a non-regression test for the future. New implementation works anyway
(smoke tested) and all existing tests pass.

Signed-off-by: Fred Bricon <fbricon@gmail.com>