Don't call Coveralls.wear_merged! if SIMPLECOV_MERGE set
Don't call Coveralls.wear_merged! if SIMPLECOV_MERGE set

MSP-12019

`Coveralls.wear_merged!` internally calls `SimpleCov.start`, so it was
causing the `rake` process's coverage to be reported, which failed the
travis-ci build.

rapid7/metasploit-cache,rapid7/metasploit-cache,rapid7/metasploit-cache,rapid7/metasploit-cache