Change required upper javax.servlet version back to 3.0.0

javax.servlet 3.1 introduces some new abstract methods, which lead to
compile errors in the corresponding subclasses in rwt.osgi
(ServletContextWrapper) and testfixture (TestRequest, TestResponse,
TestServletOutputStream...).
We should address these issues as soon as possible and then change the
version again.

Change-Id: I900bc7e12202525286b0ec4fbdff4d8dda877f7c
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>