Reset python env for CI benchmarks to 3.5. (#3336)
Reset python env for CI benchmarks to 3.5. (#3336)

#3143 restored travis jobs for environments other than 3.6, but
the benchmarks are currently set to run on 3.5, which needed
to be overridden explicitly.
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py