Disable slow Travis builds

* Changes to Travis seem to be making these builds slower, and are
  resulting in spurious timeouts. Remove the builds that fail more
  often than not until we complete our migration to another build
  system.
* Use CMake for unity builds, instead of the slow build.

.travis.yml

@@ -38,14 +38,14 @@ matrix:
   include:
     # Default BUILD is "scons".
 
-    - compiler: gcc
-      env: GCC_VER=5 BUILD=cmake TARGET=debug.nounity PATH=$PWD/cmake/bin:$PATH
+    # - compiler: gcc
+    #   env: GCC_VER=5 TARGET=debug.nounity
 
     - compiler: gcc
-      env: GCC_VER=5 TARGET=coverage
+      env: GCC_VER=5 BUILD=cmake TARGET=coverage PATH=$PWD/cmake/bin:$PATH
 
     - compiler: clang
-      env: GCC_VER=5 TARGET=debug CLANG_VER=3.8 PATH=$PWD/llvm-$LLVM_VERSION/bin:$PATH
+      env: GCC_VER=5 BUILD=cmake TARGET=debug CLANG_VER=3.8 PATH=$PWD/llvm-$LLVM_VERSION/bin:$PWD/cmake/bin:$PATH
       cache:
         directories:
           - $GDB_ROOT