Avoid duplicate Solr instances under Jetty 9

Under Jetty 9, both the solr's war file and the solr.xml jetty context 
get copied into Jetty's webapp directory. This results in two copies of 
solr running with the same configuration, one at /solr-x.x.x and one at 
/solr, which causes file lock contention.

Since all the Jetty context is being used for is to serve solr at /solr,
we can just copy the war file to webapps/solr.war (which will mount at
/solr, based on its filename), and omit the context XML file entirely.

Fixes #4.