tests: require 100% coverage again (#345)

Closes #335.

Author: tseaver

packages/google-cloud-bigtable/.coveragerc

@@ -21,7 +21,7 @@ omit =
   google/cloud/__init__.py
 
 [report]
-fail_under = 99
+fail_under = 100
 show_missing = True
 exclude_lines =
     # Re-enable the standard pragma

packages/google-cloud-bigtable/noxfile.py

@@ -198,7 +198,7 @@ def cover(session):
     test runs (not system test runs), and then erases coverage data.
     """
     session.install("coverage", "pytest-cov")
-    session.run("coverage", "report", "--show-missing", "--fail-under=99")
+    session.run("coverage", "report", "--show-missing", "--fail-under=100")
 
     session.run("coverage", "erase")