Set bounds for google-cloud-bigtable dependency. (#18)

* Set bounds for google-cloud-bigtable dependency.

* Bump version to 0.21.0.

Author: daspecster

setup.py

@@ -50,14 +50,14 @@
 
 
 REQUIREMENTS = [
-    'google-cloud-bigtable >= 0.20.0',
+    'google-cloud-bigtable >= 0.21.0, < 0.22dev',
 ]
 
 SETUP_BASE.pop('url')
 
 setup(
     name='google-cloud-happybase',
-    version='0.20.0',
+    version='0.21.0',
     description='Client library for Google Cloud Bigtable: HappyBase layer',
     long_description=README,
     url='https://github.com/GoogleCloudPlatform/google-cloud-python-happybase',