Fix changelog file location in .travis.yml

CHANGES.rst doesn't exist so it will not work.
[changelog skip]

Author: Shir0kamii

.travis.yml

@@ -10,7 +10,7 @@ script:
      | grep '\[changelog skip\]' > /dev/null; then
     echo "Skip changelog checker..."
   elif [[ "$TRAVIS_TAG" != "" ]]; then
-    ! grep -i "to be released" CHANGES.rst
+    ! grep -i "to be released" doc/changelog.rst
   else
     [[ "$TRAVIS_COMMIT_RANGE" = "" ]] || \
     [[ "$(git diff --name-only "$TRAVIS_COMMIT_RANGE" | grep doc/changelog\.rst)" != "" ]]