Explicitly choose Mono 4.0.5

- avoids future problems related to aspnet/External#48
  - e.g. when Travis updates default Mono version in `csharp` bundle

.travis.yml

@@ -8,5 +8,7 @@ install:
   - make install
   - export LD_LIBRARY_PATH="$HOME/libuvinstall/lib"
   - cd $OLDPWD
+mono:
+  - 4.0.5
 script:
   - ./build.sh --quiet verify