Remove node 7.7.0, readd node 7.6.0
Remove node 7.7.0, readd node 7.6.0

Node.js 7.7.0 contains a bug that will prevent all native modules from
building

[#140816409]

Signed-off-by: Anna Thornton <0e9b4a17e1cf85a71a11118ecdbc866e5f8ffc65@pivotal.io>

cloudfoundry/nodejs-buildpack,pkumar-appd/nodejs-buildpack,ks2software/nodejs-buildpack,ks2software/nodejs-buildpack,ks2software/nodejs-buildpack,cloudfoundry/nodejs-buildpack,oocl-ncc-itm/nodejs-buildpack,ks2software/nodejs-buildpack,oocl-ncc-itm/nodejs-buildpack,oocl-ncc-itm/nodejs-buildpack,ks2software/nodejs-buildpack,pkumar-appd/nodejs-buildpack,ks2software/nodejs-buildpack,cloudfoundry/nodejs-buildpack,ks2software/nodejs-buildpack,pkumar-appd/nodejs-buildpack,cloudfoundry/nodejs-buildpack