Reduce size of Travis build matrix to only 4 builds
Reduce size of Travis build matrix to only 4 builds

Need to keep an OSX build as most developers use OSX, but building
only node 4.0 on OSX should be sufficient for now.

sasrin/cf-abacus,cloudfoundry-incubator/cf-abacus,jsdelfino/d-abacus,kurumai/cf-abacus,nmaslarski/cf-abacus,BetaFood/cf-abacus-dev,jsdelfino/d-abacus,jsdelfino/cf-abacus-experiment,cloudfoundry-incubator/cf-abacus,danparsons/cf-abacus,BetaFood/cf-abacus-dev,KRuelY/cf-abacus,18F/cf-abacus,nmaslarski/cf-abacus,jsdelfino/x-abacus,pombredanne/cf-abacus,pombredanne/cf-abacus,nmaslarski/cf-abacus,jsdelfino/cf-abacus-experiment,KRuelY/cf-abacus,jsdelfino/x-abacus,danparsons/cf-abacus,KRuelY/cf-abacus,rajkiranrbala/cf-abacus,jsdelfino/x-abacus,rajkiranrbala/cf-abacus,jsdelfino/x-abacus,18F/cf-abacus,BetaFood/cf-abacus-dev,sasrin/cf-abacus,pombredanne/cf-abacus,jsdelfino/d-abacus,sasrin/cf-abacus,nmaslarski/cf-abacus,cloudfoundry-incubator/cf-abacus,rajkiranrbala/cf-abacus,danparsons/cf-abacus,KRuelY/cf-abacus,pombredanne/cf-abacus,18F/cf-abacus,kurumai/cf-abacus,jsdelfino/d-abacus,rajkiranrbala/cf-abacus,jsdelfino/cf-abacus-experiment