travis: attempt to make 2 build stages for cache

First stage builds riscv-tools, next stage builds verilator

.travis.yml

@@ -43,13 +43,18 @@ branches:
 
 jobs:
   include:
-    - stage: prepare cache
+    - stage: prepare cache-riscv-tools
       script:
-        - travis_wait 120 make tools verilator -C regression SUITE=none
+        - make tools -C regression SUITE=none
       before_install:
         - export CXX=g++-4.8 CC=gcc-4.8
       before_cache:
         - ls -t regression/install | tail -n+2 | sed s@^@regression/install/@ | xargs rm -rf
+    - stage: prepare cache-verilator
+      script:
+        - make verilator -C regression SUITE=none
+      before_install:
+        - export CXX=g++-4.8 CC=gcc-4.8    
     - &test
       stage: Test
       script: