add UBSAN test for clang-3.4 R-dev

Make it easier to do UBSAN test for R packages. [sanitizers](https://github.com/eddelbuettel/sanitizers) is a package to show how this test work.

Here is the [result](https://travis-ci.org/qinwf/sanitizers/builds/43941421) on Travis CI which passed on normal build, but failed on UBSAN build. 
yep, that's what i was suggesting -- but it sounds like you're one step ahead of me. :) no worries, then.