Ensure slf4j api version 1.5.6 is present

[Trello 278](https://trello.com/c/9qisbLW5)

Apache otlu (OAuth lib used by orcid and rest webapp) brings in 1.6.1, but
rest of DSpace 1.7 expects 1.5.6. Previously xmlui and dryad-rest-webapp
didn't specify a version.

According to [slf4j docs](http://www.slf4j.org/codes.html#version_mismatch),
all versions of slf4j-api are compatible, so it doesn't matter which one we
use, but we have to use the same version api and binding

> Mixing mixing different versions of slf4j-api.jar and SLF4J binding can
cause problems. For example, if you are using slf4j-api-1.7.7.jar, then you
should also use slf4j-simple-1.7.7.jar, using slf4j-simple-1.5.5.jar will not
work.

> NOTE From the client's perspective all versions of slf4j-api are compatible.
Client code compiled with slf4j-api-N.jar will run perfectly fine with
slf4j-api-M.jar for any N and M. You only need to ensure that the version of
your binding matches that of the slf4j-api.jar. You do not have to worry about
the version of slf4j-api.jar used by a given dependency in your project. You
can always use any version of slf4j-api.jar, and as long as the version of
slf4j-api.jar and its binding match, you should be fine.