Add RemoveSwitch mutator

This patch adds the support for REMOVE_SWITCH mutator set. It mutates the first 100 elements
of any swtich statement one at a time, changing it to be equivalent to default.
Move assertNoMutation to the MutatorTestBase since it is being used multiple times.
include the key in getGloballyUniqueId, and make key final.
Nudge travis ci
merge 108
switched mutator from enum to stringly typed to allow parameterised construction
fix build for java8
Merge remote-tracking branch 'origin/master' into fix/remove_conditional

Conflicts:
	pitest/src/main/java/org/pitest/mutationtest/engine/gregor/config/Mutator.java
	pitest/src/test/java/org/pitest/mutationtest/engine/gregor/config/MutatorTest.java
	pitest/src/test/java/org/pitest/mutationtest/engine/gregor/mutators/RemoveConditionalMutatorTest.java