Define paths in gitignore in a consistent way

We used a slash as a prefix in some cases but not in other cases. Now
we're defining files and folders following the gitignore rules: files
starting with a slash are only ignored in the root folder, while files
not starting with it are ignored everywhere.

IMHO it makes sense to ignore all folders named `tmp`, `.bundle`, `log`,
`.DS_Store` or `.ruby-gemset` everywhere in the source code and not only
in the root folder.

We're also adding a trailing slash to all folders for consistency.

.gitignore

@@ -5,12 +5,12 @@
 #   git config --global core.excludesfile '~/.gitignore_global'
 
 # Ignore bundler config.
-/.bundle
+.bundle/
 
 # Ignore all logfiles and tempfiles.
-/log/*
-!/log/.keep
-/tmp
+log/*
+!log/.keep
+tmp/
 
 /spec/examples.txt
 /config/database.yml
@@ -18,7 +18,7 @@
 /config/deploy-secrets.yml
 /config/maintenance.yml
 
-/coverage
+/coverage/
 
 .byebug_history
 
@@ -27,9 +27,9 @@
 
 .ruby-gemset
 
-public/sitemap.xml
-public/assets/
-public/machine_learning/data/
-public/system/
+/public/sitemap.xml
+/public/assets/
+/public/machine_learning/data/
+/public/system/
 /public/ckeditor_assets/
-storage/
+/storage/