improve handling rvm_previous_environment in __rvm_load_project_config
remove rvm_silence_path_mismatch_check_flag from rvmrc on travis for rvm-test-rvm1/path_mismatch_comment_test.sh
unset rvm_silence_path_mismatch_check_flag for travis
remove rvm_silence_path_mismatch_check_flag from rvmrc on travis for rvm-test-rvm1/path_mismatch_comment_test.sh