Can't find libxml2config.cmake on ubuntu 16.04

[danielwojtaszek]

Running install.py gives me the same error as in issue #1426 of cyclus/cyclus:

Could not find a package configuration file provided by "LibXml2" with any of the following names:

LibXml2Config.cmake
libxml2-config.cmake

[danielwojtaszek]

The fix that @bam241 used for cyclus also works for Bright-lite:

cyclus/cyclus@d4e8603#diff-af3b638bc2a3e6c650974192a53c7291