.travis.yml: propagate -w properly

r66692's `export RUBYOPT="-w"` does not seem to work.
Passing `RUBYOPT="-w"` to `make test-all`'s argument works.

Let's try having this for all environments.

(see r66690 for motivation)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66698 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -105,8 +105,6 @@ env:
   - &x86_64-linux
     name: x86_64-linux
     <<: *linux
-    env:
-      - RUBYOPT="-w" # test the same condition as RubyCI's chkbuild by this
 
   - &jemalloc
     name: --with-jemalloc
@@ -372,7 +370,7 @@ before_script:
 
 script:
   - $SETARCH make -s test TESTOPTS="${TESTOPTS=$JOBS -q --tty=no}"
-  - travis_wait 40 $SETARCH make -s test-all -o exts TESTOPTS="${TEST_ALL_OPTS=$TESTOPTS}"
+  - travis_wait 40 $SETARCH make -s test-all -o exts TESTOPTS="${TEST_ALL_OPTS=$TESTOPTS}" RUBYOPT="-w"
   - $SETARCH make -s test-spec MSPECOPT=-ff # not using `-j` because sometimes `mspec -j` silently dies
 
 # Branch matrix.  Not all branches are Travis-ready so we limit branches here.