Create build directory.

Also remove dependency on gulp-browserify and instead use browserify
directly, because that lets us use a newer version of browserify (which I
had hoped would eliminate the need to manually create the directory, but
no) and because smart people on the internet say gulp-browserify isn't
good.
Don't try to create the dir if it exists.
Merge pull request #242 from NICTA/mkdir

Fix clean build problem