Skip rev of a filename that ends in html

I expect revall('build/**') would not actually rename html files
with the md5 hash. This small change skips HTML files (but still
processes the links within the file). 

Please reject if I'm not understanding the usage of this. 
My usage has a build folder full of js, css, html and images:

		gulp.task('revision', function() {
			return gulp.src('build/**')
				.pipe(revall())
				.pipe(gulp.dest('s3'));
		});