DS-1971 Updated bte dependency to v0.9.2.4

By mistake bte-io version 0.9.2.3 had a dependency on bte-core 0.9.2.3-SNAPSHOT (http://search.maven.org/remotecontent?filepath=gr/ekt/bte/bte-io/0.9.2.3/bte-io-0.9.2.3.pom), which was producing a warning when building dspace-api. 

A new version of BTE (0.9.2.4) has been published in maven central and this pull request updates the dspace-api dependency information on BTE, so that building it does not produce this warning.