Remove references to Cache-Control public

A previous commit removed some, but missed these. Where a location
directive was using Expires to set a future expiry in conjunction
with Cache-Control public, I have replaced the time with an equal
max-age.

Furthermore, Google's web performance guide says that "public" is
implicit if there is a max-age specified.

See: https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/http-caching

doc/common-problems.md

@@ -148,7 +148,6 @@ will be a 404. The reason for this is that H5bp's basic ruleset includes, for ex
 	location ~* \.(?:jpg|jpeg|gif|png|ico|cur|gz|svg|svgz|mp4|ogg|ogv|webm|htc)$ {
 		expires 1M;
 		access_log off;
-		add_header Cache-Control "public";
 	}
 
 Which will _also_ capture any dynamic requests matching that url pattern and not
@@ -166,7 +165,6 @@ Modifying (all) location blocks as follows:
 
 		expires 1M;
 		access_log off;
-		add_header Cache-Control "public";
 	}
 
 Will make Nginx pass requests for files that don't exist to the application.

h5bp/location/cross-domain-fonts.conf

@@ -7,7 +7,6 @@ location ~* \.(?:ttf|ttc|otf|eot|woff|woff2)$ {
   # See http://wiki.nginx.org/HttpCoreModule#location
   # And https://github.com/h5bp/server-configs/issues/85
   # And https://github.com/h5bp/server-configs/issues/86
-  expires 1M;
   access_log off;
-  add_header Cache-Control "public";
+  add_header Cache-Control "max-age=2592000";
 }

h5bp/location/expires.conf

@@ -20,9 +20,8 @@ location ~* \.(?:rss|atom)$ {
 
 # Media: images, icons, video, audio, HTC
 location ~* \.(?:jpg|jpeg|gif|png|ico|cur|gz|svg|svgz|mp4|ogg|ogv|webm|htc)$ {
-  expires 1M;
   access_log off;
-  add_header Cache-Control "public";
+  add_header Cache-Control "max-age=2592000";
 }
 
 # CSS and Javascript