Add boxtree to requirements.txt to make pytential happy

requirements.txt

@@ -2,4 +2,7 @@ numpy
 git+git://github.com/inducer/modepy
 git+git://github.com/pyopencl/pyopencl
 git+git://github.com/inducer/loopy
+
+# required by pytential
+git+git://github.com/inducer/boxtree
 git+git://github.com/inducer/pytential