travis-ci: allow docker deploy for v0.11 stable repo

A conditional check for repo slug "flux-framework/flux-core" was
missed when updating .travis.yml for flux-core-v0.11 stale repo,
preventing docker tags for v0.11 from being generated.

Fix the test to check for flux-core-v0.11 to allow docker tag
deployment.

.travis.yml

@@ -117,7 +117,7 @@ before_install:
  #  Tag image if this build is on master or result of a tag:
  - |
   if test "$DOCKER_TAG" = "t" \
-    -a "$TRAVIS_REPO_SLUG" = "flux-framework/flux-core" \
+    -a "$TRAVIS_REPO_SLUG" = "flux-framework/flux-core-v0.11" \
     -a "$TRAVIS_PULL_REQUEST" = "false" \
     -a \( "$TRAVIS_BRANCH" = "master" -o -n "$TRAVIS_TAG" \); then
      export TAGNAME="${DOCKERREPO}:${IMG}${TRAVIS_TAG:+-${TRAVIS_TAG}}"