[CI] don't use conda 4.3.14 or conda-build 2.1.6
[CI] don't use conda 4.3.14 or conda-build 2.1.6

As they seem to fail with a signals-related TypeError on travis. See for example
  https://travis-ci.org/Jupyter-contrib/jupyter_nbextensions_configurator/jobs/211918641
[skip appveyor]
jcb91/jupyter_nbextensions_configurator,jcb91/jupyter_nbextensions_configurator,jcb91/jupyter_nbextensions_configurator,jcb91/jupyter_nbextensions_configurator