Move rustapi_module into examples

This is really a test module, but the Rust convention is to put
something like this under examples/, and when it lands, we can take
advantage of "Project-based Examples for Cargo Projects" - RFC link
at rust-lang/rfcs#2517

Author: pganssle

Cargo.toml

@@ -57,5 +57,4 @@ members = [
     "pyo3cls",
     "pyo3-derive-backend",
     "examples/*",
-    "tests/rustapi_module",
 ]

ci/travis/test.sh

@@ -12,6 +12,6 @@ for example in examples/*; do
   cd $TRAVIS_BUILD_DIR
 done
 
-cd tests/rustapi_module
+cd examples/rustapi_module
 tox -e py
 cd $TRAVIS_BUILD_DIR