Converter implementation is not refreshed in IDEA

Nobody has claimed this yet.

Using https://github.com/mapstruct/mapstruct-examples/tree/master/mapstruct-lombok as a reference, I tried to add a field to both `Source` and `Target` and then modified the test to cover the new field.

```
@Test
public void testMapping() {
Source s = new Source();
s.setTest( "5" );
s.setField( "5" );

Target t = Mappers.getMapper(SourceTargetMapper.class).toTarget( s );
assertEquals( 5, (long) t.getTesting() );
assertEquals( "5", t.getField() );
}
```

However this leads to the test to fail, being `t.getField()` still `null`.

Another relevant change I did (which I am not sure about) is to add `mapstruct-processor` to the `compile` configuration in `build.gradle`, ie: `compile "org.mapstruct:mapstruct-jdk8:${mapstructVersion}", "org.mapstruct:mapstruct-processor:${mapstructVersion}", "org.projectlombok:lombok:${lombokVersion}"` as per http://mapstruct.org/documentation/ide-support/

Also removing a field from both objects leads to an error, this time a `java.lang.NoSuchMethodError`.

Versions used:
- IDEA 2017.2.5 (with annotation processor manually enabled, no other customization)
- lombok 1.16.18 (instead of 1.16.14)

Use the mapstruct-lombok example and its Source, Target, and test as the reproduction; inspect build.gradle and the IDEA annotation-processor setup first. Reproduce the added-field and removed-field cases, then verify that the generated converter reflects changes and that the test no longer reports null or NoSuchMethodError.

Written by the indexing model from the issue text.