ci(travis): run codecov using npx in after_success (#106)

diff --git a/.travis.yml b/.travis.yml
index af50820..3f32cbe 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,11 +10,10 @@
 
 install:
   - npm install
-  - npm install -g codecov
 
 script:
   - npm test
   - npm run cover
 
-after_script:
-  - codecov
+after_success:
+  - npx codecov