CI: reduce number of Travis-CI jobs (#855)
CI: reduce number of Travis-CI jobs (#855)

Drop conda Py2 and virtualenv Py3.6, Py3.7.  Travis + Appveyor cover
together the supported python versions.
qwhelan/asv,airspeed-velocity/asv,spacetelescope/asv,spacetelescope/asv,qwhelan/asv,airspeed-velocity/asv,spacetelescope/asv,qwhelan/asv,airspeed-velocity/asv,qwhelan/asv,airspeed-velocity/asv,spacetelescope/asv