Pin socket2 to 0.3.9 for the time being

Version 0.3.10 of socket2 appears to contain a bug that breaks accept(2)
for Unix sockets. The issue for this can be found at
https://github.com/alexcrichton/socket2-rs/issues/38. For the time being
we will pin socket2 to 0.3.9 so our builds pass again.

vm/Cargo.lock

@@ -348,7 +348,7 @@ dependencies = [
  "rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "rayon 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "siphasher 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "socket2 0.3.10 (registry+https://github.com/rust-lang/crates.io-index)",
+ "socket2 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)",
  "time 0.1.42 (registry+https://github.com/rust-lang/crates.io-index)",
  "wepoll-binding 1.0.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "winapi 0.3.7 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -800,7 +800,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [[package]]
 name = "socket2"
-version = "0.3.10"
+version = "0.3.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "cfg-if 0.1.9 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -1067,7 +1067,7 @@ dependencies = [
 "checksum shlex 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "7fdf1b9db47230893d76faad238fd6097fd6d6a9245cd7a4d90dbd639536bbd2"
 "checksum siphasher 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "9913c75df657d84a03fa689c016b0bb2863ff0b497b26a8d6e9703f8d5df03a8"
 "checksum smallvec 0.6.10 (registry+https://github.com/rust-lang/crates.io-index)" = "ab606a9c5e214920bb66c458cd7be8ef094f813f20fe77a54cc7dbfff220d4b7"
-"checksum socket2 0.3.10 (registry+https://github.com/rust-lang/crates.io-index)" = "df028e0e632c2a1823d920ad74895e7f9128e6438cbc4bc6fd1f180e644767b9"
+"checksum socket2 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)" = "4e626972d3593207547f14bf5fc9efa4d0e7283deb73fef1dff313dae9ab8878"
 "checksum stable_deref_trait 1.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "dba1a27d3efae4351c8051072d619e3ade2820635c3958d826bfea39d59b54c8"
 "checksum strsim 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
 "checksum syn 0.15.42 (registry+https://github.com/rust-lang/crates.io-index)" = "eadc09306ca51a40555dd6fc2b415538e9e18bc9f870e47b1a524a79fe2dcf5e"

vm/Cargo.toml

@@ -43,7 +43,7 @@ version = "^0.2"
 features = ["rand"]
 [dependencies.socket2]
-version = "^0.3.9"
+version = "=0.3.9"
 features = ["unix", "reuseport"]
 [target.'cfg(not(windows))'.dependencies]