Bug fix to allow the docsystem build to complete

Before this fix, `rp-build-app` failed with:
...
+ cp -f ruby_help.tmpl ruby_help.h
+ ../../server/bin/qt-doc.rb -o ruby_help.h
/git/sonic-pi/app/server/sonicpi/lib/sonicpi/docsystem.rb:73:in `block in docs_html_map': undefined method `each_with_index' for nil:NilClass (NoMethodError)
	from /git/sonic-pi/app/server/sonicpi/lib/sonicpi/docsystem.rb:52:in `each'
	from /git/sonic-pi/app/server/sonicpi/lib/sonicpi/docsystem.rb:52:in `docs_html_map'
	from ../../server/bin/qt-doc.rb:164:in `<main>'
Update the Linux install instructions #105
Merge pull request #113 from bruceadams/linux-build

Update the Linux install instructions (and fix a build issue)