title: Gradle Liquibase plugin not using specified contexts
body: <p>We have a Gradle project which uses Liquibase, and our build file has:</p>
  
  <pre><code>buildscript {
      repositories {
          mavenCentral()
      }
      dependencies {
          classpath 'org.liquibase:liquibase-gradle-plugin:1.1.1'
          classpath 'org.liquibase:liquibase-core'
      }
  }
  
  apply plugin: 'liquibase'
  </code></pre>
  
  <p>Yet when we try to update our project via:</p>
  
  <pre><code>gradle update -Dliquibase.contexts=foobar
  </code></pre>
  
  <p>(tried with <code>--contexts</code> too)
  it seems to ignore the specified contexts and runs all the changesets instead.  </p>
  
  <p>Example:</p>
  
  <pre><code>changeSet(author: 'me', id: 'someId1', context: 'somethingElse') { // This runs, but shouldn't
  // ... 
  changeSet(author: 'me', id: 'someId2', context: 'foobar') { // Should only run this
  </code></pre>
  
  <p>The project was forked at one point, so maybe we're misinterpreting the documentation (<a href="https://github.com/tlberglund/groovy-liquibase" rel="nofollow">original</a>, <a href="https://github.com/liquibase/liquibase-groovy-dsl" rel="nofollow">new</a>) but it seems like this should work.  </p>
  
  <p>Do we need a different plugin/version?  Are we calling it wrong?</p>