Instance#clean! - do not remove downloads

[dazza-codes]

Fix #37

[dazza-codes]

If I make this change to the gem installed for hyku, then clean does not remove download .jar, only the .md5 file, e.g.

$ bundle install --path .gems
$ gvim .gems/ruby/2.4.0/gems/fcrepo_wrapper-0.8.0/lib/fcrepo_wrapper/instance.rb 
# edit that file as per this PR

$ ll ~/tmp/fcrepo_wrapper/
total 47M
-rw-r--r-- 1 dlweber users 47M May  5 14:25 fcrepo-webapp-4.7.1-jetty-console.jar
-rw-r--r-- 1 dlweber users  71 May  5 14:27 fcrepo-webapp-4.7.1-jetty-console.jar.md5

$ fcrepo_wrapper clean
cleaning /tmp/fcrepo-webapp-4.7.1-jetty-console...

$ ll ~/tmp/fcrepo_wrapper/
total 47M
-rw-r--r-- 1 dlweber users 47M May  5 14:25 fcrepo-webapp-4.7.1-jetty-console.jar

$ fcrepo_wrapper 
Starting Fedora 4.7.1 on port 8984 ...                                                                                                                            fcrepo-webapp-4.7.1-jetty-console.jar.md5: |==============================================================================================| 100% (Time: 00:00:00 )

etc. it's all good

[cbeer]

We should have a consistent approach, and wait to merge this until we resolve https://github.com/cbeer/solr_wrapper/pull/104/files.

[dazza-codes]

Applied a similar approach to the current solr_wrapper PR, may be revised again when that PR resolves.

[dazza-codes]

Note, when FileUtils.remove_* is called with the force: true, it ignores any StdErr on failures. Hence, the File.exists? checks are removed here.

[dazza-codes]

Still waiting on https://github.com/cbeer/solr_wrapper/pull/104/files