need to reattempt srs parse if srs.proj4 is undefined

[springmeyer]

Recently we moved the parsing fallback required for ESRI variant shapefile WKT into node-srs. But, node-srs only fallsback if there is an error and should not fallback only if the .proj4 value is undefined (node-srs will require other srs representations that did work). But TileMill requires the proj4 be known, so millstone still needs this fallback. Ideally the node-srs api would give a single .proj4() call, but until then we need to handle this correctly in millstone for what tilemill expects.

[springmeyer]

done in 51298eb