Check dependencies' open-source licenses (#165)

* Check dependencies' open-source licenses

Use jk1/Gradle-License-Report to verify that all
dependencies have open-source licenses.

Note that the following dependencies are not checked:

Dependencies of official Gradle plugins
Dependencies declared in buildscript block
Dependencies of jk1/Gradle-License-Report itself

buildSrc/build.gradle

@@ -52,6 +52,7 @@ repositories {
 }
 
 apply from: '../dependencies.gradle'
+apply from: '../dependency_license.gradle'
 apply from: '../java_common.gradle'
 
 sourceSets {