Do not run checkstyle as part of the gradle check task (#3985)

* Do not run checkstyle as part of the gradle check task

* Run main build before checkstyle

Author: tobiasdiez

.travis.yml

@@ -17,8 +17,8 @@ env:
   global:
     - GRADLE_OPTS=-Dorg.gradle.daemon=false
   matrix:
-    - TEST_SUITE=checkstyle
     - TEST_SUITE=check OPTIONS=modernizer
+    - TEST_SUITE=checkstyle
     - TEST_SUITE=fetcherTest
     - TEST_SUITE=databaseTest
     - TEST_SUITE=guiTest

build.gradle

@@ -395,11 +395,11 @@ checkstyle {
     // do not use other packages for checkstyle, excluding gen(erated) sources
     checkstyleMain.source = "src/main/java"
     toolVersion = '8.5'
+    
+    // do not perform checkstyle checks by default
+    sourceSets = []
 }
 
-checkstyleMain.shouldRunAfter test
-checkstyleTest.shouldRunAfter test
-
 modernizer {
     // We have more than 20 issues, which are not fixed yet. Nevertheless, we produce the modernizer output.
     // See https://github.com/andrewgaul/modernizer-maven-plugin for more information on modernizer