add missing base_branch method
add missing base_branch method

travis-ci/travis-core,GistIcon/travis-core,travis-ci/travis-core,Tiger66639/travis-core,GistIcon/travis-core,developerfm/travis-core,developerfm/travis-core,Tiger66639/travis-core,Tiger66639/travis-core,GistIcon/travis-core,developerfm/travis-core,travis-ci/travis-core