Add 32-bit build in travis-ci config (#658)

- Add 32-bit builds to Travis CI matrix
- Install gcc-multilib on Travis CI for -m32
- Add IRC use_notice to Travis CI notifications
- Set on_success and on_failure separately for IRC and email
- Ignore _build - convenient for CMake builds

Patch by Mateusz Loskot

git-svn-id: http://svn.osgeo.org/geos/trunk@3934 5242fede-7e19-0410-aef8-94bd7d2200fb