Fix tests on Rust 1.36

Some indirect dependencies dropped support for Rust 1.36 in minor
version upgrades, so we have to pin them to a version that works with
Rust 1.36.

Cargo.toml

@@ -54,10 +54,12 @@ rand_distr = "0.4"
 serde_json = "1"
 streaming-stats = "0.2"
 quantiles = "0.7"
-proptest = "1"
-# byteorder is not a direct dependency, but the MSRV of 1.4 is higher than ours.
-# Therefore, we have to enforce version 1.3.
+# Proptest 1.0 has a higher MSRV than we do
+proptest = "0.10"
+# The following are not direct dependencies, but the recent versions have a too
+# high MSRV:
 byteorder = "=1.3"
+bitflags = "=1.2"
 
 [package.metadata.docs.rs]
 # Enable certain features when building docs for docs.rs