print_versions.py fixes
#4760
#4901
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 unaffected since the system python there
   always matches the python version of the env.
3. Using subprocess meant Ctrl-C didn't interrupt the script (meh).

cc @cpcloud 
