Add CI badge and run credo in the test environment to stop needing to compile twice.

.travis.yml

@@ -5,4 +5,4 @@ otp_release:
   - 19.2
 script:
   - mix test
-  - mix credo --strict
+  - MIX_ENV=test mix credo --strict

README.md

@@ -1,5 +1,7 @@
 # Vivid
 
+[![Build Status](https://travis-ci.org/jamesotron/vivid.ex.svg?branch=master)](https://travis-ci.org/jamesotron/vivid.ex)
+
 Vivid is a simple 2D rendering library.
 
 I plan to use it to render polygons for display on a
@@ -120,4 +122,4 @@ Font use restrictions:
 
 ## Status
 
-This library is still experimental, but what few features it has work well.
+This library is still experimental, but what few features it has work well.