build.gradle: add testCompile dependency on 'mockito-core'
build.gradle: add testCompile dependency on 'mockito-core'

We will be needing it to implement the GUI tests.

CS2103AUG2016-T11-C4/main,nightism/addressbook-level4,snehasp13/main,snehasp13/main,CS2103AUG2016-T11-C4/main,nightism/addressbook-level4