reducing guava to 20.0 as 23.0 doesn't support JDK 1.7

Author: dhaval24

core/build.gradle

@@ -40,7 +40,7 @@ dependencies {
     compile ([group: 'commons-io', name: 'commons-io', version: '2.6' ])
     compile ([group: 'org.apache.commons', name: 'commons-lang3', version: '3.7'])
     compile ([group: 'org.apache.httpcomponents', name: 'httpclient', version: '4.5.3'])
-    compile ([group: 'com.google.guava', name: 'guava', version: '23.0'])
+    compile ([group: 'com.google.guava', name: 'guava', version: '20.0'])
     testCompile group: 'junit', name: 'junit', version: '4.12'
     testCompile group: 'org.mockito', name: 'mockito-all', version: '1.10.19'
     testCompile group: 'com.google.code.gson', name: 'gson', version: '2.8.2'