Updated README.md

Modified requirements:
- Java 6 is not officially supported anymore by Oracle since December 2018 => Moved to Java 7
- Piwigo >= 2.9 is now required

Author: Valou447

README.md

@@ -8,8 +8,8 @@
 
 # Requirements
 
-* A **Java** jre/jdk (version **6** or greater)
-* A **Piwigo** working **installation** (version **2.6** or greater)
+* A **Java** JRE/JDK (version **7** or greater)
+* A **Piwigo** working **installation** (version **2.9** or greater)
 
 # Quick start