CI: Pin dependencies for MSRV

Loads of deps break MSRV at the moment because the Rust ecosystem is
upgrading to edition 2021 left, right, and centre without bumping crate
versions.

Pin various crates in CI for MSRV test run.

Author: tcharding

contrib/test.sh

@@ -1,6 +1,8 @@
 
 set -xe
 
+MSRV="1\.41\.1"
+
 # Just echo all the relevant env vars to help debug Travis.
 echo "RUSTFMTCHECK: \"$RUSTFMTCHECK\""
 echo "BITCOINVERSION: \"$BITCOINVERSION\""
@@ -11,6 +13,14 @@ if [ -n "$RUSTFMTCHECK" ]; then
   cargo fmt --all -- --check
 fi
 
+# Test pinned versions (these are from rust-bitcoin pinning for 1.48).
+if cargo --version | grep ${MSRV}; then
+    cargo update -p serde_json --precise 1.0.99
+    cargo update -p serde --precise 1.0.156
+    cargo update -p quote --precise 1.0.30
+    cargo update -p proc-macro2 --precise 1.0.63
+fi
+
 # Integration test.
 if [ -n "$BITCOINVERSION" ]; then
     wget https://bitcoincore.org/bin/bitcoin-core-$BITCOINVERSION/bitcoin-$BITCOINVERSION-x86_64-linux-gnu.tar.gz