title: Cannot parametrize version in Gradle plugins block
body: <p>In Gradle when I declare plugin with hardcoded version it works well:</p>
  
  <pre><code>plugins {
      id 'org.springframework.boot' version '2.1.4.RELEASE'
  }
  </code></pre>
  
  <p>However, if I try to parametrize it then I get exception.</p>
  
  <p>Contents of <code>gradle.properties</code>:</p>
  
  <pre><code>springBootVersion=2.1.4.RELEASE
  </code></pre>
  
  <p>Contents of <code>build.gradle</code>:</p>
  
  <pre><code>plugins {
      id 'org.springframework.boot' version "$springBootVersion"
  }
  </code></pre>
  
  <p>Why the following exception is happening?</p>
  
  <pre><code>Cause: startup failed:
  build file 'build.gradle': 10: argument list must be exactly 1 literal non empty string
  
  See https://docs.gradle.org/5.2.1/userguide/plugins.html#sec:plugins_block for information on the plugins {} block
  
   @ line 10, column 5.
         id 'org.springframework.boot' version "$springBootVersion"
         ^
  
  1 error
  </code></pre>