Pr pr ver fixes

https://github.com/cpcloud/pandas/commit/12e80ca6daba67dccfdc1253e9e35956edb94e75#commitcomment-4528300

1. Path resolution was broken, so running the script outside the pandas root directory didn't work.
2. The use of `subprocess` to invoke the script from the source tree relied on system python,
ignoring the interpreter version actually used to invoke the scripts. Travis was fine since the system python there
always matches the python version of the env.

cc @cpcloud