bump mysqlclient-sys version to 0.2.4

0.2.0 do not provide enum mysql_ssl_mode.
The enum was introduces in
sgrif/mysqlclient-sys@6575414

Author: Alexei Pastuchov

diesel/Cargo.toml

@@ -17,7 +17,7 @@ byteorder = "1.0"
 chrono = { version = "0.4.1", optional = true }
 libc = { version = "0.2.0", optional = true }
 libsqlite3-sys = { version = ">=0.8.0, <0.19.0", optional = true, features = ["min_sqlite_version_3_7_16"] }
-mysqlclient-sys = { version = "0.2.0", optional = true }
+mysqlclient-sys = { version = "0.2.4", optional = true }
 pq-sys = { version = "0.4.0", optional = true }
 quickcheck = { version = "0.9", optional = true }
 serde_json = { version = ">=0.8.0, <2.0", optional = true }