collect_minidumps.py does not support python 2.6

Python 2.6 is the default python version shipped with CentOS 6.6 and
the minidump collection script needs to run there, too. However, the
tarfile library version shipped with python 2.6 does not support using
TarFile objects as context managers. To fix this we should wrap
tarfile.TarFile in contextlib.closing to support python 2.6, too.