pzhokhov test dockerfiles for ubuntu 14.04 and 18.04 (#1168)
* pass device_id=-1 when rendering mujoco in rgb_array mode

* trying ubuntu18.04 build

* use travis build matrix

* use travis build matrix

* use travis build matrix

* use fix 18.04 installation steps

* use fix 18.04 installation steps

* try py3 instead of py35

* trying the 14.04 build; cleaning up the 16.04 package list

* fixes to dockerfile.14.04

* fixes to dockerfile.14.04

* fixes to dockerfile.14.04

* fixes to dockerfile.14.04

* fixes to dockerfile.14.04

* fixing system package list for 14.04

* 16.04 build working

* clean up test.dockerfile.16.04

* adding python-numpy system package

* add patchelf

* remove python-numpy from 16.04 dockerfile

* 14.04 does not use mujoco (patchelf problem)

* cleaning up 18.04 dockerfile

* added comment to 14.04 dockerfile

* fix syntax in 14.04 dockerfile

* being fancy and switching from apt-get to apt command in 18.04 dockerfile

* update README.rst to use apt (instead of apt-get) on ubuntu 18.04
Latest commit 69f5f98 Sep 21, 2018