Enforces Python2 usage

[HalisCz]

Should fix #38

[nickjj]

Isn't it idiomatic Python to assume python is 2 where as python3 is specifically 3?

In this case, the problem is that Ansigenome doesn't support 3 at the moment but I'm not sure I agree with forcing 2 in the mean time. A more long term approach would be to get Ansigenome running on both 2 and 3. Problem is, I don't have 3 installed atm.

[HalisCz]

Isn't it idiomatic Python to assume python is 2 where as python3 is specifically 3?

It should be, according to PEP 394, but as PEP 394 says, on Arch Linux is /usr/bin/python linked to /usr/bin/python3
I made some checks across distributions and this is the compatibility of the ansigenome v.0.5.6:

• Arch - /usr/bin/python points to python3
• Centos 7.2 - /usr/bin/python points to python2
• Debian 8 - /usr/bin/python points to python2
• Fedora 23 - in minimal image is python3 installed, python doesn't exists
• Gentoo - /usr/bin/python points to python3
• Ubuntu 14.04 - in minimal image is python3 installed, python doesn't exists

In this case, the problem is that Ansigenome doesn't support 3 at the moment but I'm not sure I agree with forcing 2 in the mean time. A more long term approach would be to get Ansigenome running on both 2 and 3. Problem is, I don't have 3 installed atm.

I disagree. The problem I've encountered is that my system (Gentoo) links python to python3, but contains python2 too. So with this pull request I am able to run ansigenome.