travis: Reduce stages to flaky builds (gcov, tsan) and everything else

Separating the non-flaky builds (asan, normal builds, lint) into
separate stages simply slowed down overall CI turnaround.  Since none of
the builds rely on the output of others, reducing the stages increases
the opportunities for parallel builds.

.travis.yml

@@ -51,14 +51,13 @@ env:
 
 jobs:
   include:
-    - stage: sanitizers
+    - stage: normal builds
       os: linux
       compiler: clang
       env: >
         CLANG_SANITIZER=ASAN_UBSAN
         CMAKE_FLAGS="$CMAKE_FLAGS -DPREFER_LUA=ON"
-    - stage: normal builds
-      os: linux
+    - os: linux
       compiler: gcc
       env: FUNCTIONALTEST=functionaltest-lua
     - os: linux
@@ -73,8 +72,7 @@ jobs:
     - os: osx
       compiler: gcc
       osx_image: xcode7.3  # macOS 10.11
-    - stage: lint
-      os: linux
+    - os: linux
       env: CI_TARGET=lint
     - stage: Flaky builds
       os: linux