Rise the upper javax.servlet version to 4.0.0 in testfixture

The version shipped by the Eclipse Platform in Mars (Platform 4.5) is
javax.servlet_3.1.0 and exports javax.servlet;version="3.1.0".

Change-Id: Ib8bca85d6e870fc28446561e6447ff8e9127b209
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>