Intermittent RSTcheckBear 35s timeout

See https://travis-ci.org/jayvdb/docker-coala-base/builds/202889527 and https://travis-ci.org/jayvdb/docker-coala-base/builds/202614783

```
E Traceback (most recent call last):
E File "/coala/coalib/bears/Bear.py", line 250, in execute
E result = self.run_bear_from_section(args, kwargs)
E File "/coala/coalib/bears/Bear.py", line 243, in run_bear_from_section
E return self.run(*args, **kwargs)
E File "/coala/coalib/bearlib/abstractions/Linter.py", line 555, in run
E cwd=self.get_config_dir())
E File "/coala/coalib/misc/Shell.py", line 142, in run_shell_command
E ret = p.communicate(stdin)
E File "/usr/lib64/python3.5/subprocess.py", line 1068, in communicate
E stdout, stderr = self._communicate(input, endtime, timeout)
E File "/usr/lib64/python3.5/subprocess.py", line 1702, in _communicate
E ready = selector.select(timeout)
E File "/usr/lib64/python3.5/selectors.py", line 367, in select
E fd_event_list = self._poll.poll(timeout)
E File "/usr/lib/python3.5/site-packages/pytest_timeout.py", line 110, in handler
E timeout_sigalrm(item, timeout)
E File "/usr/lib/python3.5/site-packages/pytest_timeout.py", line 243, in timeout_sigalrm
E pytest.fail('Timeout >%ss' % timeout)
E File "/usr/lib/python3.5/site-packages/_pytest/runner.py", line 542, in fail
E raise Failed(msg=msg, pytrace=pytrace)
E Failed: Timeout >35.0s
```

Start with coalib/bears/Bear.py, coalib/bearlib/abstractions/Linter.py, and coalib/misc/Shell.py, following the pytest_timeout traceback. Reproduce the intermittent RSTcheck failure using the linked Travis builds, then trace why the subprocess exceeds the 35-second test timeout. Done means the relevant test completes reliably within the timeout.

Written by the indexing model from the issue text.