Build cs variant on Travis, but allow failure.

With this script the "default" value of RACKETCS_SUFFIX is effectively ""
instead of "cs" as in the make file.
This is a minor incompatibility/unexpected behavior that will get eventually
solved when the real default changes to "".

.travis.yml

@@ -1,6 +1,7 @@
 sudo: false
 language: c
 matrix:
+  include:
   - os: osx
     compiler: clang
     env: PATH=./racket/bin:$PATH
@@ -21,7 +22,10 @@ matrix:
     compiler: gcc
     env: PATH=./racket/bin:$PATH RACKET_CONFIGURE_ARGS="--disable-jit --disable-places
       --disable-futures --disable-extflonum"
-  allow-failures:
+  - os: linux
+    compiler: clang
+    env: PATH=./racket/bin:$PATH TARGET="cs"
+  allow_failures:
   - os: linux
     compiler: clang
     env: PATH=./racket/bin:$PATH TARGET="cs"
@@ -29,7 +33,13 @@ before_script:
 - git config --global user.email "travis-test@racket-lang.org"
 - git config --global user.name "Travis Tester"
 script:
-- make $TARGET CPUS="2" PKGS="racket-test db-test unstable-flonum-lib net-test" CONFIGURE_ARGS_qq="$RACKET_CONFIGURE_ARGS"
+- make $TARGET  \
+       CPUS="2"  \
+       PKGS="racket-test db-test unstable-flonum-lib net-test"  \
+       CONFIGURE_ARGS_qq="$RACKET_CONFIGURE_ARGS"  \
+       RACKETCS_SUFFIX="$RACKETCS_SUFFIX"
+- which racket
+- racket -v
 - raco test -l tests/racket/test
 - racket -l tests/racket/contract/all
 - raco test -l tests/json/json