Fixed Travis, missing import in export.py

IPython/nbconvert/exporters/export.py

@@ -18,6 +18,7 b' from functools import wraps'
 from IPython.nbformat.v3.nbbase import NotebookNode
 from IPython.config import Config
 
+from .exporter import Exporter
 from .templateexporter import TemplateExporter
 from .html import HTMLExporter
 from .slides import SlidesExporter