Automatically include correct mocha version in multiverse suites

This should fix issues where mocha sometimes gets required before
test-unit, leading to intermittent 'unexpected invocation' failures in
CI. To opt-out of automatic mocha inclusion in a multiverse suite,
just add a call to omit_mocha! to your suite Envfile.
Add exception_class_name, alias exception_class

@exception_class is used outside of the agent in core so it is necessary to
alias exception_class_name temporarily. When the code in core that references
exception_class expecting the string class name has been changed to reference
exception_class_name this alias should be removed.
Deprecate exception_class
Merge pull request #75 from newrelic/RUBY-1109

Add exception_class_name, alias exception_class
Merge branch 'dev' into release
CHANGELOG entry for RUBY-1070
RUBY-1096 Disable RequestSampler by default for now
RUBY-1096 Remove test that was asserting the RequestSampler was enabled by default