Update php:5.5 references to be php:5.6

Fixes #142

php/README.md

@@ -72,7 +72,7 @@ We provide two convenient scripts named `docker-php-ext-configure` and `docker-p
 
 For example, if you want to have a PHP-FPM image with `iconv`, `mcrypt` and `gd` extensions, you can inheriting the base image that you like, and write your own `Dockerfile` like this:
 
-	FROM php:5.5-fpm
+	FROM php:5.6-fpm
 	# Install modules
 	RUN apt-get update && apt-get install -y \
 	    libmcrypt-dev libpng12-dev libfreetype6-dev libjpeg62-turbo-dev \

php/content.md

@@ -58,7 +58,7 @@ We provide two convenient scripts named `docker-php-ext-configure` and `docker-p
 
 For example, if you want to have a PHP-FPM image with `iconv`, `mcrypt` and `gd` extensions, you can inheriting the base image that you like, and write your own `Dockerfile` like this:
 
-	FROM php:5.5-fpm
+	FROM php:5.6-fpm
 	# Install modules
 	RUN apt-get update && apt-get install -y \
 	    libmcrypt-dev libpng12-dev libfreetype6-dev libjpeg62-turbo-dev \