changed README.markdown installation guide to the correct gem name
changed README.markdown installation guide to the correct gem name

testerhome/social-share-button,marvindpunongbayan/social-share-button,sofarsounds/social-share-button,Kaizeras/social-share-button-foundation,marvindpunongbayan/social-share-button,codeodor/social-share-button,jogiranjith/social-share-button,Matrixbirds/icon-social-share-button,drselump14/social-share-button,hoangnghiem/social-share-button