Restored FFI to working order (#28)

* Restored FFI to working order

* Disabled incompatible tests, fixed string conversion

* Use extension_loaded instead of checking PHP version

* Added better checks for FFI

* Do not build FFI deps if no FFI is present

ci-test.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+set -ex
+
+HAS_FFI=$(php -m | grep FFI | wc -l | xargs)
+
+make test-extension
+
+if [[ $HAS_FFI -gt 0 ]]; then
+  make test-ffi
+fi