remove abstract declaration of 'retrieve' which duplicated interface declaration
add specific php version 5.3.3 to validate issue #84

- add composer install option '--prefer-source', see issue travis-ci/travis-ci#1385
Merge pull request #85 from alexmmm/fix-duplicate-abstract-declaration

remove abstract declaration of 'retrieve' which duplicated interface declaration
Merge pull request #86 from alexmmm/extend-travis-conf

add specific php version 5.3.3 to travis build to validate issue #84