Update copyright year in docs

spring-boot-docs/src/main/asciidoc/index-docinfo.xml

 <productname>Spring Boot</productname>
 <releaseinfo>{spring-boot-version}</releaseinfo>
 <copyright>
-	<year>2013-2014</year>
+	<year>2013-2015</year>
 </copyright>
 <legalnotice>
 	<para>