CLN: Move print_versions into pandas proper.
Moves print_versions under util so that it can be called from buildbot
(and so we can tell people to run `from pandas.util.print_versions import show_versions; show_versions()`
when trying to debug)

cc @yarikoptic

Any objections? This is minor...
 fine by me
 I checked that running `python print_versions.py` works and this runs appropriately on Travis [i.e. it prints versions]
 @yarikoptic - ask and ye shall receive, you can use:

``` python
from pandas.util.print_versions import show_versions
show_versions()
```
