[MRG] Configure coverage creation and checks (scikit-optimize#757)

* Update .codecov.yml

* Create .coveragerc

Author: betatim

.codecov.yml

@@ -1,3 +1,10 @@
+# show coverage in CI status, not as a comment. 
+comment: off
 coverage:
-  ignore:
-    - "skopt/tests/*"
+  status:
+    project:
+      default:
+        target: auto
+    patch:
+      default:
+        target: 20% 

.coveragerc

@@ -0,0 +1,6 @@
+[run]
+omit =
+    # this file comes from versioneer and we don’t test it
+    */_version.py
+    # these are our tests, don't count them towards coverage
+    skopt/tests/*