Limit yamllint version on python 2.6.

(cherry picked from commit 55aec8ebc9)

test/runner/requirements/constraints.txt

@@ -2,4 +2,5 @@ coverage >= 4.2, != 4.3.2 # features in 4.2+ required, avoid known bug in 4.3.2
 pywinrm >= 0.2.1 # 0.1.1 required, but 0.2.1 provides better performance
 pylint >= 1.5.3, < 1.7.0 # 1.4.1 adds JSON output, but 1.5.3 fixes bugs related to JSON output
 sphinx < 1.6 ; python_version < '2.7' # sphinx 1.6 and later require python 2.7 or later
+yamllint < 1.8 ; python_version < '2.7' # yamllint 1.8 and later require python 2.7 or later
 isort < 4.2.8 # 4.2.8 changes import sort order requirements which breaks previously passing pylint tests