Switch to jacoco maven plugin

Currently this project uses cobertura plugin for code coverage.
Cobertura requires a separate test phase for calculation of the coverage, which increases the build time.
Jacoco on the other hand is integrated into the standard phases (uses agent to connect to surefire process), so the build process won't need to run the unit tests twice - should speed up the builds in travis and rultor.

Jacoco maven plugin documentation: http://eclemma.org/jacoco/trunk/doc/maven.html