Ignore empty ROLLBAR_ENV

[gr8bit]

If ENV variables are not set, they appear as nil in ruby when accessed. If they are empty, they appear as en empty string "" to ruby. An empty strings is true when used with || operator in ruby. As we want to have an environment set for Rollbar, we should use .presence to ignore empty ROLLBAR_ENV variables and use the Rails.env instead.

$ ENV_FOO=test ENV_BAR= be rails c
Loading development environment (Rails 5.0.3)
2.3.3 :001 > ENV['ENV_FOO']
 => "test" 
2.3.3 :002 > ENV['ENV_BAR']
 => "" 
2.3.3 :003 > ENV['ENV_BAZ']
 => nil 

[gr8bit]

Failures are travis-internal:

Finished in 10.42 seconds (files took 5.08 seconds to load)
400 examples, 0 failures
Randomized with seed 14174
/home/travis/build/rollbar/rollbar-gem/lib/rollbar/delay/thread.rb:58:in `rescue in block in configure_exit_handler': unable to reap all threads within 3 seconds (Rollbar::Delay::Thread::TimeoutError)
	from /home/travis/build/rollbar/rollbar-gem/lib/rollbar/delay/thread.rb:52:in `block in configure_exit_handler'
/home/travis/.rvm/rubies/ruby-2.0.0-p598/bin/ruby -I/home/travis/.rvm/gems/ruby-2.0.0-p598/gems/rspec-core-3.6.0/lib:/home/travis/.rvm/gems/ruby-2.0.0-p598/gems/rspec-support-3.6.0/lib /home/travis/.rvm/gems/ruby-2.0.0-p598/gems/rspec-core-3.6.0/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb failed

[rokob]

LGTM, the travis result is unrelated as you say.