travis deduping on prs
closes #12438 
ok, in theory this should work :>

I have a test commit up and the original PR. when the original runs it should cancel almost immediately (not 100% sure the osx build will work as this relies on the system python, but will see) https://travis-ci.org/pandas-dev/pandas/jobs/213816512

success! Cool! you also may notice that builds now take about 2hrs when in full-caching. so if extensions are rebuilt then this goes up to about 3. and when I touch ci/* it deletes the cache for 2 builds :< (and thus takes about 3 as well )