[INFR-1205] add support for branch/master envars with tests

[aks]

This PR adds two new options: -e ENVAR=VALUE and -E ENVAR=VALUE, which can be provided multiple times to declare envars and values to be assigned for the test branch and the reference branch, respectively.

It also adds a -M option as a short-hand for --migrations.

This allows perf_check to test changes that depend on specific envar values.

There is a new example test to verify the use of the envar support at lines 52-74 in server_spec.rb.

All specs pass.

[aks]

The TravisCI failure is in the master branch -- which suggests that the test_app is missing the "railties" gem. Not sure how to fix the "master" branch?

can't find executable rails for gem railties. railties is not currently included in the bundle, 
perhaps you meant to add it to your Gemfile? (Gem::Exception)

[sudara]

A test case knows when its context is :reference https://github.com/rubytune/perf_check/blob/master/lib/perf_check/test_case.rb#L119

Maybe we can ask it here and send the appropriate env var into server.restart https://github.com/rubytune/perf_check/blob/master/lib/perf_check.rb#L115

[aks]

@siggymcfried:

• in regard to the new Rakefile -- yes, without it, it's not possible to do bundle exec rake spec.
• in regard to the rails_helper.rb -- without it, we get errors in running rake spec without it (even though it's empty)