Update tests to make them pass on travis

[pmuller]

Failing tests :

• tests/test_integration.py:97: test_stat
• test_integration.py:104: test_partitions
• test_integration.py:107: test_softirqs
• test_integration.py:110: test_interrupts
• test_integration.py:116: test_diskstats

See https://travis-ci.org/pmuller/procfs/jobs/31210663 for details.

[FedericoCeratto]

I wonder if it's worth running functional tests that are reading the /proc directory on Travis hosts. Maybe we shouldn't rely on /proc on those hosts and make the functional tests optional. We could add proper unit tests (using mocked files)