Introduced Drone provider and better test coverage for ContinuousIntegrationInfoPlugin

[rpalcolea]

Adds DroneCI (https://drone.io/) support

Also adds tests to make sure that a jar populates manifest attributes by continuous integration info plugin. This introduces JUnit System Rules (https://github.com/stefanbirkner/system-rules) to allow setting environment variables in the test that get clear out when the test finishes. I'm not a big fan of introducing more dependencies but this might be helpful for the tests. Also, it might be worth to extract getEnvironmentVariable from AbstractContinuousIntegrationProvider (https://github.com/nebula-plugins/gradle-info-plugin/blob/master/src/main/groovy/nebula/plugin/info/ci/AbstractContinuousIntegrationProvider.groovy#L27) in a way that could be mock on a simple fashion.

[coveralls]

Coverage increased (+2.9%) to 72.508% when pulling 62b9043 on rpalcolea:drone-support into 0469cc7 on nebula-plugins:master.

[sghill]

Thanks @rpalcolea! Would you mind rebasing to resolve the conflicts and we'll get this merged?

[rpalcolea]

Hi @sghill, the branch is now up-to-date with master. Thanks!