Actually add the Stack build target

Just added the build to the list of allowed failures before,
not to the actual list of things to build :-)

.travis.yml

@@ -58,9 +58,7 @@ matrix:
      os: osx
    - env: GHCVER=8.0.1 SCRIPT=bootstrap
      os: osx
-  allow_failures:
-   - env: GHCVER=head
-     sudo: required
+
    - env: GHCVER=via-stack SCRIPT=stack STACKAGE_RESOLVER=lts
      os: linux
      addons:
@@ -68,6 +66,11 @@ matrix:
             packages:
                 - libgmp-dev
 
+  allow_failures:
+   - env: GHCVER=head
+     sudo: required
+   - env: GHCVER=via-stack SCRIPT=stack STACKAGE_RESOLVER=lts
+
  # TODO add PARSEC_BUNDLED=YES when it's so
  # It seems pointless to run head if we're going to ignore the results.
  #- GHCVER=head