Added HHVM release 2.4.0
Added HHVM release 2.4.1
Updated jenkins to LTS 1.532.2 and latest 1.551
Added dev-db/pinba-engine-9999
Updated dev-php/pinba-extension-9999
Updated HHVM dependencies

- Raised minimum boost version to 1.49
- Added libxslt dependency
- Added imagemagick dependency