Bump core version number (#3864)

Author: lukesneeringer

packages/google-cloud-bigtable/setup.py

@@ -51,7 +51,7 @@
 
 
 REQUIREMENTS = [
-    'google-cloud-core >= 0.26.0, < 0.27dev',
+    'google-cloud-core >= 0.27.0, < 0.28dev',
     'google-gax>=0.15.7, <0.16dev',
 ]