Changes mapnik dependency in windshaft module to use node-mapnik 1.4.13
which is based on mapnik-2.3.x
Merge branch 'master' into mapnik-2.3.x
Starts using mapnik 2.3.x via windshaft upgrade
Modify travis configuration to be able to compile node-mapnik with mapnik 2.3.x
Merge pull request #213 from CartoDB/mapnik-2.3.x

Starts using mapnik 2.3.x by upgrading windshaft dependency