Changed round_trip converter to Python converter instead of strtod
As @mdickinson noted in #8044.

pls do a perf check, assuming ok, ping on green
 @amras1 progress?
 @jreback any idea what [this](https://travis-ci.org/amras1/pandas/jobs/36404385#L1602) could be?
 looks like u rebased (which was the excel issue / that fixed a few days ago)

why was this failing on 2.6?

looks good now
 Silly mistake when I wrote it, I had `<` instead of `>=` for the Python >= 2.7 check.
 thanks for the fix!
 And a big thank you from me, too!
 No problem!
