Make travis use `dist: bionic` for the docs build.
Make travis use `dist: bionic` for the docs build.

This means we use an updated doxygen to match the updated config.

sourcefrog/librsync,sourcefrog/librsync,librsync/librsync,dbaarda/librsync,sourcefrog/librsync,librsync/librsync,dbaarda/librsync,sourcefrog/librsync,dbaarda/librsync,librsync/librsync,dbaarda/librsync,librsync/librsync