Improve travis.yml

- Added new compilers and OS X images
- Option to run SelfTest under Valgrind
- Merge "Debug" and "Release" configurations into one run
-- This saves apt setup and cmake download step per compiler, 60-90s
- Fix C++14 compilation under Clang 3.8 and up

appveyor.yml

@@ -1,6 +1,10 @@
 # version string format -- This will be overwritten later anyway
 version: "{build}"
 
+branches:
+  except:
+    - /dev-travis.+/
+
 os:
   - Visual Studio 2017
   - Visual Studio 2015