Add support for different groovy versions when using spock

[pchudzik]

spock1.1 is required but groovy version isn't fixed.

Add support for different groovy versions (2.3, 2.4, 2.0) the same versions spock does support - http://mvnrepository.com/artifact/org.spockframework/spock-core

[leonard84]

Just FYI, spock 1.2 will only support Groovy 2.4+