Minor installation doc tweaks

 - Update path to solr example config (probably due to changes to solr)
 - Specify full path to openspendingjs
 - Don’t specify solr version number
 - Although “Pong!” is better than “Pong.” it does say “Pong.”
Installation doc tweak

The path to openspendingjs is what is relevant here (since the user could
clone the openspendingjs repo somewhere else)
Merge pull request #621 from andylolz/620-update-installation-doc

620 update installation doc