Update travis build for docs sync

.travis.yml

 language: java
-install: mvn install -q -U -DskipTests=true -Dmaven.test.redirectTestOutputToFile=true
+before_install: git config user.name "$GIT_NAME" && git config user.email "$GIT_EMAIL"
+  && git config credential.helper "store --file=.git/credentials" && echo "https://$GH_TOKEN:@github.com" > .git/credentials
+install:
+  - mvn install -q -U -DskipTests=true -Dmaven.test.redirectTestOutputToFile=true
+  - ./src/main/asciidoc/ghpages.sh
 script: mvn --settings .settings.xml deploy -nsu -Dmaven.test.redirectTestOutputToFile=true
 env:
   global:
-  - secure: JXDn4awZAYbhxPV9PIn0I+wUv+PwLJ5TLWFVW2q9rQqeJVDJd3KB0WRXSFOi03g/rGFbN0j+JCqgJPW/I91yEwC55AWbr2DG9fB7HuyQe6OKNT7qZkmoDxrfkYhaN7OLwcNf7vP4kXwzKqIbmSLfsrF1M/aVAPdXw9u6TPL+9i0=
-  - secure: cwftjc6405DztK+P2oJYsxSZWYqozsAWPaLPzuqq/1bTjp+3WAPkV+/OGWiZDW7Exd/z958McQMSONIab/CgLvfenrYc38ErjKstdovyduYs905aVitlUk19dOiH35qJgeFl9+6zNWIOB4OysWsBmdnmtbk1avM/v/Ove3bJvcE=
+    - GIT_NAME="Dave Syer"
+    - GIT_EMAIL=dsyer@pivotal.io
+    - secure: JXDn4awZAYbhxPV9PIn0I+wUv+PwLJ5TLWFVW2q9rQqeJVDJd3KB0WRXSFOi03g/rGFbN0j+JCqgJPW/I91yEwC55AWbr2DG9fB7HuyQe6OKNT7qZkmoDxrfkYhaN7OLwcNf7vP4kXwzKqIbmSLfsrF1M/aVAPdXw9u6TPL+9i0=
+    - secure: cwftjc6405DztK+P2oJYsxSZWYqozsAWPaLPzuqq/1bTjp+3WAPkV+/OGWiZDW7Exd/z958McQMSONIab/CgLvfenrYc38ErjKstdovyduYs905aVitlUk19dOiH35qJgeFl9+6zNWIOB4OysWsBmdnmtbk1avM/v/Ove3bJvcE=
+    - secure: mk2cdXuMTQKL3bIyFsE+dAmxHN5PvKKptPhsV4TjeBq7nYGqchnfUNRhRabCZNj7FpRP5G8CyXmWWb9RQX8gYzlEAjD+XIOPB2PX1+nphaWRjoInXIVFFzHzAb5tVdrvO/NVAdd9dbFqq7Eoj4QrS+5bRR8V/NtK1TFfsOyWFgY=

src/main/asciidoc/ghpages.sh

+#!/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