change interpreter from python to python2

[Radek Podgorny]

some distributions (arch among others) have python3 as default interpreter.

please change the interpreter in the header to call specifically python2. otherwise it fails with wrong syntax (python2-style prints). see the attached patch.

this should be backwards compatible.

[Olivier Girard (eolinwen)]

Thanks a lot for this patch. It will be put in a next milestone after being Checked.
On a side note, there aren't a lot of distribution which are using Python3 by default. And I have not yet checked on Manjaro Linux (a derivative of Archlinux) this issue. But Soon.

[Radek Podgorny (rpodgorny)]

uh, oh... ...why? :-O

[Andy Finch (fincha)]

Openshot 2.0 uses Python3, the old version that uses python2 will not be updated anymore.