Try a lambda instead of a method reference for java 8 support

Android's new toolchain does not seem to be able to handle those yet.

Author: rubengees

library/src/main/java/me/proxer/library/api/EnumRetrofitConverterFactory.java

@@ -16,7 +16,7 @@ final class EnumRetrofitConverterFactory extends Converter.Factory {
     public Converter<?, String> stringConverter(final Type type, final Annotation[] annotations,
                                                 final Retrofit retrofit) {
         if (((Class<?>) type).isEnum()) {
-            return (Converter<Enum<?>, String>) ProxerUtils::getApiEnumName;
+            return (Converter<Enum<?>, String>) value -> ProxerUtils.getApiEnumName(value);
         }
 
         return null;