title: CircleCI: env variable depending on branch
body: <p>Id like to set different value for environment variable in circle.yml depending on branch name.Is it possible?</p>
  
  <p>What i have:</p>
  
  <pre><code>machine:
      environment:
           MYVAR:"VAL1"
  </code></pre>
  
  <p>What id like to have is to be able detect type of the branch (master or not) and assign respective value to the variable?</p>