upgrade to llvm 6 toolchain

 - Uses new packages for llvm 6.0.0 (and sub-packages) mapbox/mason#600 (@AllieOop)
 - Uses bundled pyyaml so that should no longer be needed when using >= llvm 6 (refs #106)

Author: Dane Springmeyer

mason-versions.ini

@@ -1,8 +1,8 @@
 [headers]
 protozero=1.6.1
 [compiled]
-clang++=5.0.1
-clang-tidy=5.0.1
-clang-format=5.0.1
-llvm-cov=5.0.1
+clang++=6.0.0
+clang-tidy=6.0.0
+clang-format=6.0.0
+llvm-cov=6.0.0
 binutils=2.30