stbt lint: Remove compatibility code for pylint < 1.5
stbt lint: Remove compatibility code for pylint < 1.5

As of the previous commit, stbt-lint probably doesn't work with
pylint < 1.5.

This affects Ubuntu 14.04 (pylint 1.1) but that'll reach end of life in
5 months so I don't want to put any effort into supporting it.

Ubuntu 16.04 has pylint 1.5.2.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester