Fix flaky tests

For some reason running Gradle 3.0 and 5.6.1 causes the integration
tests to fail locally.

Travis builds broke with the default OS changing to Xenial, so going
back to Trusty to see if that stabilizes it.

.travis.yml

@@ -1,3 +1,4 @@
+dist: trusty
 language: java
 cache:
   directories:

src/test/groovy/com/github/benmanes/gradle/versions/DifferentGradleVersionsSpec.groovy

@@ -55,7 +55,7 @@ final class DifferentGradleVersionsSpec extends Specification {
         dependencies {
           compile 'com.google.inject:guice:2.0'
         }
-        
+
         dependencyUpdates.resolutionStrategy {
           componentSelection {
             all {
@@ -101,7 +101,7 @@ final class DifferentGradleVersionsSpec extends Specification {
       '5.3.1',
       '5.4.1',
       '5.5.1',
-      '5.6.1'
+      '5.6',
     ]
   }
 
@@ -136,14 +136,14 @@ final class DifferentGradleVersionsSpec extends Specification {
         dependencies {
           compile 'com.google.inject:guice:2.0'
         }
-        
+
         dependencyUpdates.gradleReleaseChannel="${gradleReleaseChannel}"
-        
+
         """.stripIndent()
 
     when:
     def result = GradleRunner.create()
-      .withGradleVersion('3.0')
+      .withGradleVersion('3.3')
       .withProjectDir(testProjectDir.root)
       .withArguments('dependencyUpdates')
       .forwardStdError(srdErrWriter)