Point to travis-core master branch

https://github.com/travis-ci/travis-core/pull/519 has been merged,
so there is no need for a special branch

Gemfile

@@ -5,7 +5,7 @@ ruby '2.1.7' if ENV.key?('DYNO')
 
 gem 's3',              github: 'travis-ci/s3'
 
-gem 'travis-core',     github: 'travis-ci/travis-core', branch: 'ha-feature-debug-endpoint'
+gem 'travis-core',     github: 'travis-ci/travis-core'
 gem 'travis-support',  github: 'travis-ci/travis-support'
 gem 'travis-config',   '~> 0.1.0'
 gem 'travis-sidekiqs', github: 'travis-ci/travis-sidekiqs', require: nil

Gemfile.lock

@@ -50,8 +50,7 @@ GIT
 
 GIT
   remote: git://github.com/travis-ci/travis-core.git
-  revision: 501f31a40af589b5ea8a677e7f8b067949a322b2
-  branch: ha-feature-debug-endpoint
+  revision: fb39f8af5628444e2d7f5893a9f09fde7b0796e2
   specs:
     travis-core (0.0.1)
       actionmailer (~> 3.2.19)