Update travis build for docs sync

.settings.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<settings>
+  <servers>
+	<server>
+	  <id>repo.spring.io</id>
+	  <username>${env.CI_DEPLOY_USERNAME}</username>
+	  <password>${env.CI_DEPLOY_PASSWORD}</password>
+	</server>
+  </servers>
+</settings>

src/main/asciidoc/ghpages.sh

@@ -0,0 +1,46 @@
+#!/bin/bash -x
+
+git remote set-url --push origin `git config remote.origin.url | sed -e 's/^git:/https:/'`
+
+if ! (git remote set-branches --add origin gh-pages && git fetch -q); then
+    echo "No gh-pages, so not syncing"
+    exit 0
+fi
+
+if ! [ -d target/generated-docs ]; then
+    echo "No gh-pages sources in target/generated-docs, so not syncing"
+    exit 0
+fi
+
+# Stash any outstanding changes
+###################################################################
+git diff-index --quiet HEAD
+dirty=$?
+if [ "$dirty" != "0" ]; then git stash; fi
+
+# Switch to gh-pages branch to sync it with master
+###################################################################
+git checkout gh-pages
+
+for f in target/generated-docs/*; do
+    file=${f#target/generated-docs/*}
+    if ! git ls-files -i -o --exclude-standard --directory | grep -q ^$file$; then
+        # Not ignored...
+        cp -rf $f .
+        git add -A $file
+    fi
+done
+
+git commit -a -m "Sync docs from master to gh-pages"
+
+# Uncomment the following push if you want to auto push to
+# the gh-pages branch whenever you commit to master locally.
+# This is a little extreme. Use with care!
+###################################################################
+git push origin gh-pages
+
+# Finally, switch back to the master branch and exit block
+git checkout master
+if [ "$dirty" != "0" ]; then git stash pop; fi
+
+exit 0