Fix tokio feature dependencies

[antis81]

Closes #475

[coveralls]

Pull Request Test Coverage Report for Build 6943511016

• 0 of 8 (0.0%) changed or added relevant lines in 1 file are covered.
• 14 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-0.08%) to 94.796%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tox/src/lib.rs	0	8	0.0%

Files with Coverage Reduction	New Missed Lines	%
tox_core/src/net_crypto/crypto_connection.rs	1	98.76%
tox_core/src/net_crypto/mod.rs	1	98.47%
tox_core/src/state_format/old.rs	1	85.11%
tox_packet/src/dht/errors.rs	1	75.0%
tox_core/src/onion/client/paths_pool.rs	2	85.47%
tox_packet/src/onion/mod.rs	2	93.62%
tox_core/src/relay/links.rs	3	98.3%
tox_node/src/node_config.rs	3	72.92%

Totals
Change from base Build 5022320978:	-0.08%
Covered Lines:	16796
Relevant Lines:	17718

---

💛 - Coveralls