change format of modernizr task

grunt task `build` fails out of the box due to changes in how grunt-modernizr works. See https://github.com/Modernizr/grunt-modernizr#config-options and https://github.com/Modernizr/grunt-modernizr#devfile-string. The proposed changes allowed me to run `grunt` on a newly generated app with no warnings.
Fixes #317
Merge branch 'patch-modernizr' of github.com:morenoh149/generator-webapp into patch-modernizr