Ignore files under "/shared" directory and not other subdirectories that contains that name

It's working fine now but by making it more explicit we can make tools
like 'The Silver Searcher' to parse properly the .gitignore file

.gitignore

@@ -46,4 +46,4 @@ rails_best_practices_output.html
 tmp/
 vendor/bundle/*
 builds/*
-shared/*
+/shared/*