Rename flexboxlegacy test names properly to master

According to latest Modernizr source the flexboxlegacy test is not named `flexbox-legacy` but `flexboxlegacy` (http://modernizr.com/downloads/modernizr-latest.js) which throws errors in the build process. I’ve change the occurrences.
This should fix https://github.com/Modernizr/grunt-modernizr/issues/50 then. Please test it again before merging but I'd really love to see this issue resolved finally.