Test docker build in CI

Signed-off-by: Vadim Markovtsev <vadim@sourced.tech>

.travis.yml

@@ -31,7 +31,6 @@ matrix:
   fast_finish: true
   allow_failures:
     - go: tip
-    - go: 1.11.x
 
 stages:
   - test
@@ -81,6 +80,10 @@ script:
 
 jobs:
   include:
+    - stage: test
+      language: generic
+      env: DOCKER_BUILD=1
+      script: docker build -t srcd/hercules .
     - stage: deploy
       os: osx
       osx_image: xcode9.3