README fix (#78)

* Add service tag to README send span

* Update version

Author: joannatk

README.md

@@ -332,6 +332,7 @@ wavefront_sender.send_span(
     span_id=UUID("0313bafe-9457-11e8-9eb6-529269fb1459"),
     parents=[UUID("2f64e538-9457-11e8-9eb6-529269fb1459")],
     follows_from=None, tags=[("application", "Wavefront"),
+                             ("service", "istio"),
                              ("http.method", "GET")],
     span_logs=None)
 ```

setup.py

@@ -15,7 +15,7 @@
 
 setuptools.setup(
     name='wavefront-sdk-python',
-    version='1.7.7',
+    version='1.7.8',
     author='Wavefront by VMware',
     author_email='[email protected]',
     url='https://github.com/wavefrontHQ/wavefront-sdk-python',