update Travis and README for manual rustc 1.22.0 steps

README.md

@@ -53,6 +53,12 @@ freenode.
 ## Minimum Supported Rust Version (MSRV)
 This library should always compile with any combination of features on **Rust 1.22**.
 
+Because some dependencies have broken the build in minor/patch releases, to
+compile with 1.22.0 you will need to run the following version-pinning command:
+```
+cargo update -p cc --precise "1.0.41" --verbose
+```
+
 ## Installing Rust
 Rust can be installed using your package manager of choice or
 [rustup.rs](https://rustup.rs). The former way is considered more secure since

contrib/test.sh

@@ -45,5 +45,12 @@ then
     cargo new dep_test
     cd dep_test
     echo 'bitcoin = { path = "..", features = ["use-serde"] }' >> Cargo.toml
+
+    # Pin `cc` for Rust 1.22
+    if [ "$TRAVIS_RUST_VERSION" = "1.22.0" ]; then
+        cargo generate-lockfile --verbose
+        cargo update -p cc --precise "1.0.41" --verbose
+    fi
+
     cargo test --verbose
 fi