Print validated files to stderr (#391)

Adding a pipe to tee /dev/stderr so that we can see which files are being
passed to validate-krew-manifest. Though, we print all files before validating
any of them, but still useful since mostly 1 plugin is updated at a time.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

.travis.yml

@@ -27,5 +27,5 @@ install: true # skip go get ./...
 script:
   - export PATH="$PATH:$HOME/.krew/bin";
     git diff --name-only --diff-filter=AM $TRAVIS_BRANCH...HEAD |
-      grep -E 'plugins/.*' |
+      grep -E 'plugins/.*' | tee /dev/stderr |
       xargs -I _ $HOME/bin/validate-krew-manifest -manifest _