Use docker go image

By using the official go docker image there is no need to install go
manually and this saves time pulling the ruby image which is not being
used anyway. Also, some time is save now not all packages need to be
installed to install Go.

.gitlab-ci.yml

-before_script:
-  - apt-get update -qq && apt-get install -y curl unzip bzip2
-  - curl -O https://storage.googleapis.com/golang/go1.6.2.linux-amd64.tar.gz
-  - echo 'e40c36ae71756198478624ed1bb4ce17597b3c19d243f3f0899bb5740d56212a  go1.6.2.linux-amd64.tar.gz' | shasum -c -a256 -
-  - rm -rf /usr/local/go && tar -C /usr/local -xzf go1.6.2.linux-amd64.tar.gz
-  - export PATH=/usr/local/go/bin:$PATH
+image: golang:1.6.2
 
 test:
-  script: make clean test
+  script: 
+    - apt-get update -qq && apt-get install -y unzip bzip2
+    - make test
+