adds PHP 5.5 to build matrix
adds PHP 5.5 to build matrix

curzio-della-santa/serializer,mpoiriert/serializer,paali/serializer,jms-serializer/serializer,zalatnaicsongor/serializer,Klinche/serializer,gkocakaya/serializer,nhaynes/serializer,jmcclell/serializer,apa-opensource/serializer,vslinko-forks/serializer,kuborgh/serializer,Bluestart83/serializer,nick4fake/serializer,julienverrecchia/serializer,p-nowak/serializer,crazycodr/serializer,Nilead/serializer,jockri/serializer,leberknecht/serializer,j-arnaiz/serializer,scasei/serializer,sesch/serializer,brettminnie/serializer,potherca-contrib/serializer,schmittjoh/serializer,ahsanity/serializer,ArteGEIE/serializer,marcoleong/serializer,Avazanga1/Serializer,Soullivaneuh/serializer,thenetexperts/serializer,megazoll/serializer,sloba88/serializer,alexdesignworks/serializer,progrupa/serializer