Use "bundle exec" in front of jekyll, to make it more robust (see https://stackoverflow.com/questions/6317980/you-have-already-activated-x-but-your-gemfile-requires-y).
diff --git a/build.sh b/build.sh
old mode 100644
new mode 100755
index b494149..6242fb4
--- a/build.sh
+++ b/build.sh
@@ -1,6 +1,6 @@
 set -e
 cd site
-jekyll build -d ../content_tmp
+bundle exec jekyll build -d ../content_tmp
 # Remove dangling references to the DejaVu fonts in the Javadoc stylesheets
 sed -i "" "/^@import.*dejavu.css/d; s/'DejaVu[^']*',//g" ../content_tmp/javadoc/*/stylesheet.css
 COMMIT_HASH=`git rev-parse HEAD`