Add a travis build step that tests sandbox changes (JuliaPackaging#129)

Now that the sandbox is part of the rootfs, our CI currently
does not test changes to the sandbox anymore. Add a build
step that uses the sandbox from this repo.

.travis.yml

@@ -23,11 +23,26 @@ env:
   matrix:
     - BINARYBUILDER_USE_SQUASHFS=true
     - BINARYBUILDER_USE_SQUASHFS=false
-
+    
 cache:
   directories:
     - deps/downloads
 
+# Add a test job that builds a new version of the sandbox.
+jobs:
+  include:
+    - stage: test
+      julia: 0.6
+      env:
+        - BINARYBUILDER_USE_SQUASHFS=false
+        - TEST_SANDBOX=true
+
+script:
+   - julia -e 'Pkg.clone(pwd())'
+   - julia -e 'Pkg.build("BinaryBuilder")'
+   - if [ ${TEST_SANDBOX:-false} = true ]; then julia -e 'cd(Pkg.dir("BinaryBuilder","deps")); run(`gcc -o root/sandbox sandbox.c`)'; fi
+   - julia --check-bounds=yes -e 'Pkg.test("BinaryBuilder", coverage=true)'
+
 # Ironic.  He could provide binaries for others but not himself...
 addons:
   apt: