Rely on codecov for 100% project coverage (#264)

.travis.yml

@@ -30,5 +30,5 @@ script:
 after_script:
   - if [ -f .coverage ]; then
       python -m pip install codecov;
-      codecov;
+      codecov --required;
     fi

codecov.yml

@@ -0,0 +1,8 @@
+coverage:
+    status:
+        patch:
+            default:
+                target: '100'
+        project:
+            default:
+                target: '100'

setup.cfg

@@ -16,4 +16,4 @@ line_length = 88
 multi_line_output = 3
 
 [tool:pytest]
-addopts = --cov=httpx --cov=tests --cov-report=term-missing --cov-fail-under=100
+addopts = --cov=httpx --cov=tests --cov-report=term-missing