bump dependency and plugin versions
clean key later to avoid corruption of URIs containing dashes fixes #21
Support prefix from maven. fixes #23

Prefix specification was already available from CLI and programmatic
methods, so just needed to patch it into the Vocabulary class and link
it through in the mojo
Manage version for httpclient-cache that is pulled in from jsonld-java
use spaces instead of tabs in README.md
Merge pull request #22 from ansell/master

Fixes issue: Bump some dependencies for release and fix URIs containing dashes. THX @peter, now we are ready for a new release.