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 

> 1. Path resolution was broken, so running the script outside the pandas root directory didn't work.

I tried this. It worked just fine on my end.

> Using subprocess meant Ctrl-C didn't interrupt the script (meh).

C'mon, _really_?

No. 2 makes sense.
 ``` python
python: can't open file 'pandas/util/print_versions.py': [Errno 2] No such file or directory
Traceback (most recent call last):
  File "./print_versions.py", line 16, in <module>
    show_versions()
  File "./print_versions.py", line 12, in show_versions
    return subprocess.check_call(['python', sv_path])
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['python', 'pandas/util/print_versions.py']' returned non-zero exit status 2
```

didn't work on my end.

` `
I know, that's what the _meh_ was for.
