Specify minimal version of the coverage module for coverage_report.py

BTW: I can't find related changelog lines, but it seems that the issue 2708 is fixed too (for ver >= 3.4).

I'm not sure that 3.4 is the *minimal* required version (this one is coming with Debian Wheezy).  But for some older versions this script just breaks down (e.g. for Debian Squeeze):
```
$ ./bin/coverage_report.py 
Traceback (most recent call last):
  File "./bin/coverage_report.py", line 104, in <module>
    make_report(source_dir, **options.__dict__)
  File "./bin/coverage_report.py", line 66, in make_report
    cov = coverage.coverage()
  File "/usr/lib/python2.6/dist-packages/coverage.py", line 303, in __init__
    raise CoverageException("Only one coverage object allowed.")
coverage.CoverageException: Only one coverage object allowed.
```