MikeHeiber/KMC_Lattice: Build System Update

This minor update adds clang compiler support to the makefile and updates the Travis CI build matrix to test clang compilation and improve detection of the compiler installed on the user's system. The Travis CI build matrix is also updated to use the Ubuntu Linux xenial or bionic distributions instead of trusty in order to fix a bug with code coverage reporting. There are no changes to the underlying code in this release.