Fix README.md

3 jobs for master in 59 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
failed build #962458

00:00:59

 
  Test
skipped code_quality #962460
allowed to fail
skipped test #962459
 
Name Stage Failure
failed
build Build
Building Heroku-based application using gliderlabs/herokuish docker image...
invalid reference format
invalid reference format
invalid argument "/master:b37335436565f607fa5046c47888126071856e3e" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.
Running after_script
Uploading artifacts for failed job
Authenticating with credentials from /root/.docker/config.json
ERROR: Job failed: exit code 1