uritemplate 0.1.0 is not compatible with atdgen 2.16
fails with
```
=== ERROR while compiling uritemplate.0.1.0 ==================================#
context 2.4.0~alpha1~dev | linux/x86_64 | ocaml-base-compiler.4.14.2 | file:///home/opam/opam-repository
path ~/.opam/4.14/.opam-switch/build/uritemplate.0.1.0
command ~/.opam/opam-init/hooks/sandbox.sh build dune runtest -p uritemplate -j 255
exit-code 1
env-file ~/.opam/log/uritemplate-7-995b8f.env
output-file ~/.opam/log/uritemplate-7-995b8f.out
File "test/dune", line 3, characters 31-37:
3 | (libraries uritemplate oUnit atdgen))
^^^^^^
Error: Library "atdgen" not found.
-> required by _build/default/test/uritemplate_test.exe
-> required by alias test/runtest in test/dune:2
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>