Pin pyo3-pack to known working version

Author: danieldk

.appveyor.yml

@@ -8,7 +8,7 @@ install:
   - set PATH=%PATH%;%USERPROFILE%\.cargo\bin
   - rustc -V
   - cargo -V
-  - cargo install pyo3-pack
+  - cargo install pyo3-pack --vers 0.5.0
 
 build_script:
 - pyo3-pack build --release

.travis.yml

@@ -32,7 +32,7 @@ install:
       source venv/bin/activate
       pip install cffi virtualenv
     fi
-  - cargo install pyo3-pack
+  - cargo install pyo3-pack --vers 0.5.0
   - rustup default nightly-2019-02-07
   - rustup component add rustfmt