Use Codecov instead of Coveralls

As we do in most other repos.

.travis.yml

-os:
-  - linux
-  - osx
+sudo: false
 
-language: go
 
+language: go
 go:
-    - 1.7
-
-install: true
-
-before_install:
-  - make deps
+  - 'tip'
   
+install:
+  - go get github.com/whyrusleeping/gx
+  - go get github.com/whyrusleeping/gx-go
+  - gx install --global
 script:
-  - go vet
-  - $GOPATH/bin/goveralls -service="travis-ci"
+  - gx test -v -race -coverprofile=coverage.txt -covermode=atomic .
+  
+after_success:
+  - bash <(curl -s https://codecov.io/bash)
 
 cache:
   directories:
     - $GOPATH/src/gx
 
 notifications:
-  email: false
+email: false
+