Remove allprojects block around target versions

This Gradle project has no subprojects, so the allprojects block is
unnecessary.

build.gradle

@@ -12,10 +12,8 @@ wrapper {
   distributionType = Wrapper.DistributionType.ALL
 }
 
-allprojects {
 sourceCompatibility = 1.6
 targetCompatibility = 1.6
-}
 
 dependencies {
   testCompile 'junit:junit:4.8.2'