Add version-dependent display filter flag

#38 refers to a problem with using display filters. This patch asks TShark for its version, then chooses a display filter flag that's appropriate for that version.

Consider this script:

```python
from __future__ import print_function
import pyshark

tshark_version = pyshark.tshark.tshark.get_tshark_version()
print(tshark_version)

parameters = pyshark.capture.capture.Capture(display_filter='something').get_parameters()
print(parameters)
```

When my config.ini points to version 1.6.7, I get output:

```
(1, 6, 7)
['-R', 'something']
```

When it points to version 1.10.6, I get output:

```
(1, 10, 6)
['-Y', 'something']
```
Note that this breaks support for py2.6 (check_output does not exist). It's not a big deal but we should be aware of it.