Fix build-time Refaster template loading (#121)

When using the system classloader, `RefasterCheckTest` is able to
successfully load the Refaster templates from the classpath, causing the
tests to pass. However, when during a build the Java compiler loads
`RefasterCheck`, the templates on the annotation processor classpath are
_not_ exposed through the system classloader.

error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/RefasterCheck.java

@@ -195,7 +195,7 @@ private static ImmutableListMultimap<String, CodeTransformer> loadAllCodeTransfo
 
   private static ImmutableSet<ResourceInfo> getClassPathResources() {
     try {
-      return ClassPath.from(ClassLoader.getSystemClassLoader()).getResources();
+      return ClassPath.from(RefasterCheck.class.getClassLoader()).getResources();
     } catch (IOException e) {
       throw new UncheckedIOException("Failed to scan classpath for resources", e);
     }