add clang 6 debug build to travis
add clang 6 debug build to travis

real-logic/Aeron,EvilMcJerkface/Aeron,real-logic/Aeron,real-logic/Aeron,EvilMcJerkface/Aeron,EvilMcJerkface/Aeron,real-logic/Aeron,mikeb01/Aeron,mikeb01/Aeron,galderz/Aeron,mikeb01/Aeron,galderz/Aeron,galderz/Aeron,mikeb01/Aeron,galderz/Aeron,EvilMcJerkface/Aeron