Update Gradle wrapper and make tests use older Gradle version to test compatibility

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.1.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.4.1-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

src/test/groovy/net/fabricmc/loom/EmptyBuildFunctionalTest.groovy

@@ -37,6 +37,7 @@ class EmptyBuildFunctionalTest extends Specification {
 				.withProjectDir(testProjectDir.root)
 				.withArguments('build')
 				.withPluginClasspath()
+				.withGradleVersion("5.1.1")
 				.build()
 
 		then:

src/test/groovy/net/fabricmc/loom/MixinBuildFunctionalTest.groovy

@@ -59,9 +59,10 @@ class MixinBuildFunctionalTest extends Specification {
 
 		when:
 		def result = GradleRunner.create()
-				.withPluginClasspath()
 				.withProjectDir(testProjectDir.root)
 				.withArguments('build')
+				.withPluginClasspath()
+				.withGradleVersion("5.1.1")
 				.build()
 
 		then:

src/test/groovy/net/fabricmc/loom/SimpleBuildFunctionalTest.groovy

@@ -47,6 +47,7 @@ class SimpleBuildFunctionalTest extends Specification {
 				.withProjectDir(testProjectDir.root)
 				.withArguments('build')
 				.withPluginClasspath()
+				.withGradleVersion("5.1.1")
 				.build()
 
 		then: