Fix for LibSass error compiling keyframe @mixin

Hey Tom, great handy library!  Was just using this with node / libsass compiler and hit upon an error, fix is included which should work with all compilers.

Error:

    Warning: bower_components/animate-sass/helpers/mixins:35: error: non-terminal statement or declaration must end with ';'
     Use --force to continue.

    Aborted due to warnings.

More information here : https://github.com/sindresorhus/grunt-sass/issues/33#issuecomment-34324000