Upgrade to PowerMock 1.6.1

The Platform would like to move to JUnit 4.12 but cannot do so at the
moment as Spring Security uses a version of PowerMock which is
incompatible with JUnit 4.12. This commit updates Spring Security to use
PowerMock 1.6.1 with is compatible with JUnit 4.12.

gradle/javaprojects.gradle

@@ -21,7 +21,7 @@ ext.hsqlVersion = '2.3.1'
 ext.slf4jVersion = '1.7.5'
 ext.logbackVersion = '0.9.29'
 ext.cglibVersion = '2.2'
-ext.powerMockVersion = '1.5.1'
+ext.powerMockVersion = '1.6.1'
 ext.servletApiVersion = '3.0.1'
 ext.seleniumVersion = '2.33.0'
 ext.groovyVersion = '2.0.5'